Youtube Thumbnail Design Agent Skill

> Install the belt CLI skill: npx skills add belt-sh/cli

YouTube Thumbnail Design

Create high-CTR YouTube thumbnails with AI image generation via inference.sh CLI.

Quick Start

> Requires inference.sh CLI (belt). Install instructions

belt login

# Generate a thumbnail
belt app run falai/flux-dev-lora --input '{
  "prompt": "YouTube thumbnail style, close-up of a person with surprised excited expression looking at a glowing laptop screen, vibrant blue and orange color scheme, dramatic studio lighting, shallow depth of field, high contrast, cinematic",
  "width": 1280,
  "height": 720
}'

Specifications

| Spec | Value |

|------|-------|

| Dimensions | 1280 x 720 px (minimum) |

| Recommended | 1920 x 1080 px |

| Aspect ratio | 16:9 |

| Max file size | 2 MB |

| Formats | JPG, GIF, PNG |

The 120px Test

Your thumbnail appears at roughly 120px wide on mobile — that's how most viewers first see it.

At 120px, viewers must be able to identify:

• The mood/emotion (from colors and expression)
• The general subject (from composition)
• The text (if any — only if large enough)

Test: view your thumbnail at 120px width. If it's a muddy blur, redesign.

Safe Zones

┌─────────────────────────────────────────────┐
│                                             │
│   ✅ SAFE FOR TEXT AND KEY ELEMENTS         │
│                                             │
│                                             │
│                                             │
│                                             │
│                                       ┌───┐ │
│                                       │ ⏱ │ │ ← Timestamp overlay
│                              ┌────────┴───┘ │    (bottom-right)
│   ┌────┐                     │  DURATION    │
│   │ CH │ Chapter marker      └──────────────│
└───┴────┴────────────────────────────────────┘
     ↑ Bottom-left: chapter/progress markers

Avoid placing critical elements in:

• Bottom-right corner (video duration timestamp)
• Bottom-left corner (chapter markers, progress bar)
• Extreme edges (cropping varies by device)

Color Strategy

High-Contrast Pairs That Work

| Combination | Mood | Best For |

|-------------|------|----------|

| Yellow + Black | Urgency, attention | Tech, business, lists |

| Red + White | Energy, excitement | Entertainment, reactions |

| Blue + Orange | Professional contrast | Education, tutorials |

| Green + White | Growth, money | Finance, success stories |

| Purple + Yellow | Premium, creative | Design, art, creativity |

| White + Dark | Clean, minimal | Luxury, minimalist channels |

Color Rules

• Background and text/subject should be complementary or high-contrast
• Avoid same-temperature colors touching (red on orange = mud)
• Use 3 colors maximum per thumbnail
• Saturate more than real life — thumbnails compete with bright UI

Text on Thumbnails

When to Use Text

• Lists/numbers: "7 Tips", "Top 10"
• Strong opinions: "STOP Doing This"
• Results: "$10K in 30 Days"
• Comparisons: "vs" between two things

When NOT to Use Text

• The video title already says it (redundant)
• The emotion/visual tells the story
• You can't make it large enough to read at 120px

Text Rules

| Rule | Reason |

|------|--------|

| Max 6 words | Readability at thumbnail size |

| Min 60pt equivalent | Must be legible at 120px width |

| Bold sans-serif font | Thin fonts disappear at small sizes |

| Contrast stroke/shadow | Ensures readability on any background |

| No small text | If it's not readable small, cut it |

Face Expression Psychology

Thumbnails with faces get higher CTR than faceless thumbnails. Expression matters:

| Expression | CTR Impact | Best For |

|------------|-----------|----------|

| Surprise/shock | Highest | Reaction, reveal, discovery content |

| Curiosity | High | Tutorial, how-to, tips |

| Excitement | High | Unboxing, reviews, announcements |

| Concern/worry | Medium-high | Warning, mistake, problem content |

| Confidence | Medium | Expert advice, authority content |

| Neutral | Lowest | Avoid unless your brand is minimalist |

Face Composition Rules

• Face should fill 30-50% of the thumbnail
• Eyes looking toward the text or subject (directs viewer attention)
• Eyes looking at camera = connection. Eyes looking at object = curiosity.
• Place face on one side (usually left), text or subject on the other

# Generate a face-forward thumbnail
belt app run falai/flux-dev-lora --input '{
  "prompt": "close-up portrait of a man with genuinely surprised expression, mouth slightly open, raised eyebrows, looking at camera, left side of frame, vibrant teal background, dramatic rim lighting, YouTube thumbnail style, high contrast, cinematic",
  "width": 1280,
  "height": 720
}'

# Generate a face-looking-at-subject thumbnail
belt app run bytedance/seedream-4-5 --input '{
  "prompt": "person looking amazed at a glowing holographic chart showing upward growth, dramatic blue and green lighting, right side profile view, dark background, tech aesthetic, high energy",
  "size": "2K"
}'

Thumbnail Patterns by Content Type

Tutorial / How-To

belt app run falai/flux-dev-lora --input '{
  "prompt": "overhead flat lay of organized workspace with laptop showing code editor, colorful sticky notes, coffee cup, clean bright background, professional setup, tutorial style composition, warm lighting",
  "width": 1280,
  "height": 720
}'

Before/After

belt app run falai/flux-dev-lora --input '{
  "prompt": "split composition, left side dark and messy disorganized desk, right side bright clean organized minimalist workspace, dramatic contrast between chaos and order, clear dividing line in center, high contrast",
  "width": 1280,
  "height": 720
}'

Product Review / Comparison

belt app run falai/flux-dev-lora --input '{
  "prompt": "two products facing each other with dramatic lighting and sparks between them, competition battle concept, dark background with colorful rim lighting, versus comparison style, high energy, product photography",
  "width": 1280,
  "height": 720
}'

Listicle / Number

belt app run falai/flux-dev-lora --input '{
  "prompt": "dynamic arrangement of 7 different colorful objects floating in space against dark gradient background, each item distinct and clearly separated, energetic composition, vibrant saturated colors, studio lighting",
  "width": 1280,
  "height": 720
}'

A/B Testing

Test one variable at a time:

| Variable | Test A vs B |

|----------|-------------|

| Face vs No face | Same composition, with/without person |

| Expression | Surprise vs curiosity |

| Color scheme | Warm vs cool palette |

| Text vs No text | With/without text overlay |

| Background | Bright vs dark |

| Composition | Left-facing vs right-facing subject |

# Generate variant A
belt app run falai/flux-dev-lora --input '{
  "prompt": "..., bright yellow background, ...",
  "width": 1280, "height": 720
}' --no-wait

# Generate variant B (same prompt, different background)
belt app run falai/flux-dev-lora --input '{
  "prompt": "..., dark navy background, ...",
  "width": 1280, "height": 720
}' --no-wait

Thumbnail Checklist

• [ ] 1280x720 minimum (1920x1080 preferred)
• [ ] Under 2MB file size
• [ ] Passes the 120px squint test
• [ ] No critical elements in bottom-right (timestamp) or bottom-left (chapter)
• [ ] Max 3 colors, high contrast
• [ ] Text (if any) is max 6 words, bold, with contrast stroke
• [ ] Face expression matches content energy (if applicable)
• [ ] Doesn't duplicate the video title
• [ ] Stands out from surrounding thumbnails (check your niche)
• [ ] Works on both light and dark YouTube backgrounds

Common Mistakes

| Mistake | Problem | Fix |

|---------|---------|-----|

| Too much text | Unreadable at thumbnail size | Max 6 words or no text |

| Low contrast | Disappears in the feed | Use complementary colors |

| Cluttered composition | Eye doesn't know where to look | One focal point |

| Generic stock photo feel | No personality, gets skipped | Authentic expressions, unique angles |

| Tiny details | Lost at 120px | Bold, simple shapes |

| Same style every video | Viewer fatigue | Vary within brand guidelines |

| Misleading thumbnail | Kills trust, hurts retention | Match the actual content |

Related Skills

npx skills add inference-sh/skills@ai-image-generation
npx skills add inference-sh/skills@image-upscaling
npx skills add inference-sh/skills@prompt-engineering

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