YouTube Thumbnail Skill

This skill enables generation of high-performing YouTube thumbnails optimized for click-through rate (CTR). Thumbnails are designed to spark curiosity, complement titles, and compel viewers to click.

Thumbkit

You have access to Thumbkit, a CLI tool for generating and editing high-performing YouTube thumbnails. You MUST use Thumbkit to generate and edit all thumbnails.

Using Thumbkit is REQUIRED for you to complete your task. Assume Thumbkit has been installed as a uv tool and is available globally on the user's system. If Thumbkit is not installed, you MUST install it before proceeding.

Testing Installation

Test if Thumbkit is installed:

thumbkit --version

Installation

uv tool install https://github.com/kenneth-liao/thumbkit.git

Upgrading

uv tool upgrade thumbkit

Thumbkit Documentation

To access the full CLI reference documentation, run the following command:

thumbkit docs

🚨 REQUIRED READING 🚨

The following documents are MANDATORY READING before generating ANY thumbnail.

1. references/design-requirements.md - The design requirements are what enable you to generate high click-through-rate thumbnails through proven strategies.
2. references/prompting-guidelines.md - Thumbnails are generated using Gemini image models (NanoBanana). The prompting guidelines will enable you to get more predictable and consistent results from the Gemini image models.
3. You MUST read the complete Thumbkit CLI reference documentation by running thumbkit docs.

It's a MANDATORY REQUIREMENT that you follow both the design requirements and prompting guidelines in order to generate high converting thumbnails. Failure to do so will result in a failed task.

Reference Images

With both generating and editing thumbnails, you can include reference images. Examples include but are not limited to thumbnail templates, headshots, icons, logos, or images for style transfer. See the Prompting Guidelines above for more.

All reference images MUST be passed using absolute paths.

Using Official Logos

If using company logos, use actual images by passing the absolute path to the image files instead of simply describing them. Nanobanana does not know what common company logos look like.

If a company logo is not locally available, you can search for it online and download it using curl, then pass the absolute path to the downloaded image in the prompt. Save all downloaded images to ./youtube/downloads/, making the dir if it doesn't exist.

Common Mistakes to Avoid

❌ WRONG: "create the Claude AI logo (an orange C shape)" ✓ CORRECT: Pass the actual logo file as a reference image

❌ WRONG: "add the Python logo" ✓ CORRECT: Use /absolute/path/to/python-logo.png as a reference image

Proven Designs

If you know the video title/subject, you can search youtube for related OUTLIER videos. Focus on OUTLIERS because they are already proven to work well and very likely have high click-through rates. Videos are outliers if they have a high amount of views compared to the subscriber count of the channel. For example, if a video has 10,000+ views from a channel with less than 5000 subscribers, that video is an outlier.

Once you have a list of outlier videos, you can access their thumbnails and use them to style transfer.

Here's an URL example of a standard definition thumbnail for a video with id "rmvDxxNubIg": https://img.youtube.com/vi/rmvDxxNubIg/sddefault.jpg. You can download the image using curl to ./youtube/downloads/outliers/, and then read the image file to understand it. Use it as a reference image when using for style transfer.

Workflows

Generating Thumbnail Concepts

Once you have generated an initial thumbnail concept or prompt, you MUST use the Thumbnail Reviewer agent to review the concept and provide feedback. The reviewer will provide a critique and suggest improvements. Consider the reviewer's feedback and incorporate it before proceeding. DO NOT go through the generate-review-regenerate loop more than ONCE.

Optimizing Thumbnails

Because you can edit a base image with Thumbkit, you can iteratively modify/improve a previously generated thumbnail. For example, if you've generated a thumbnail but want to change the color scheme:

thumbkit edit \
  --base absolute/path/to/original-thumbnail.png \
  --prompt "Keep everything the same, but change the text color to bright red."

Always review generated thumbnails to ensure they meet the complete design requirements and original intent. If not, you can iterate by editing the original thumbnail. DO NOT go through the generate-review-regenerate loop more than ONCE.

User Assets

If the user has specified any local assets (e.g. thumbnail templates, headshots, icons, logos, etc.) in their local context, bias towards incorporating them into the thumbnail when relevant. For example, Thumbnails with actual people in them have been shown to perform better.