Image Generation Skill

Generate illustration images for blog posts, documentation, and technical articles using Google Gemini's Nano Banana 2 (gemini-3.1-flash-image-preview) model.

Prerequisite: GEMINI_API_KEY must be set in environment variables.

When to Use

User asks to generate an illustration, diagram, or concept image
User is writing an article and needs visual explanations for concepts or workflows
User explicitly asks for AI-generated images

Step 1: Determine the Image Requirements

Before generating, clarify:

What to illustrate — the concept, architecture, flow, or scene
Language — default to English for both prompt and text in image. Only use other languages if the user explicitly requests it
Save location — see "Output Path" section below
Style/color preferences — if user has specific needs, otherwise use defaults

Step 2: Craft the Prompt

Default Style Prefix (always prepended unless user overrides)

The script automatically prepends this style prefix:

Use a clean, modern color palette with soft tones. Minimalist flat illustration style with clear visual hierarchy. Professional and polished look suitable for technical blog articles. No photorealistic rendering. No excessive gradients or shadows.

Prompt Writing Guidelines

Be specific and descriptive about the visual elements
For technical concepts: describe the components, their relationships, and layout
For architecture diagrams: list the layers/components and how they connect
For flow diagrams: describe the steps and direction of flow
If text labels are needed in the image, spell them out explicitly in the prompt
Default language is English. Only write the prompt and request in-image text in another language if the user specifically asks

Example Prompts

Architecture diagram:

A system architecture diagram showing: User sends query to an API Gateway,
which routes to a Vector Database (labeled "Milvus") and an LLM service.
The Vector Database returns relevant documents, which are combined with the
original query and sent to the LLM for final response generation.
Arrows show data flow direction. Each component is a rounded rectangle with
an icon and label.

Concept illustration:

A visual comparison of keyword search vs semantic search. Left side shows
keyword search with exact word matching (highlighted matching words).
Right side shows semantic search with a brain icon understanding meaning
and connecting related concepts with dotted lines. A dividing line separates
the two approaches.

Step 3: Generate the Image

Run the script:

python ${CLAUDE_SKILL_ROOT}/scripts/generate_image.py \
  --prompt "your prompt here" \
  --output "/path/to/save/image.png"

Default Parameters

Parameter	Default	Notes
Model	`gemini-3.1-flash-image-preview` (Nano Banana 2)	Fastest with good quality
Aspect ratio	`3:2`	Landscape (width:height = 3:2), ideal for article illustrations
Image size	`1K`	Good balance of quality and cost
Style	Minimal, clean, soft tones	Auto-prepended to prompt
Language	English	Prompt and in-image text

Available Options

--model          Model ID (gemini-3.1-flash-image-preview, gemini-3-pro-image-preview, gemini-2.5-flash-image)
--aspect-ratio   1:1, 2:3, 3:2, 3:4, 4:3, 4:5, 5:4, 9:16, 16:9, 21:9, etc.
--image-size     512, 1K, 2K, 4K
--style-prefix   Custom style prefix (replaces the default)
--no-style       Skip default style prefix entirely

When to Change Defaults

Scenario	Change
User wants higher quality	`--image-size 2K`
User wants best quality	`--model gemini-3-pro-image-preview --image-size 2K`
Social media banner	`--aspect-ratio 16:9`
Portrait/vertical image	`--aspect-ratio 3:4` or `--aspect-ratio 9:16`
Square image (icon, avatar)	`--aspect-ratio 1:1`
User has their own style	`--style-prefix "your style"` or `--no-style`
Non-English content	Write prompt in target language, no parameter change needed

Step 4: Determine Output Path

Follow this priority order to decide where to save the generated image:

Priority 1: Context from Current Conversation

If the user is working on a specific markdown file or article:

Check where existing images in that article are stored (look for ![ image references in the .md file)
Save the new image in the same directory as the existing images
Use a descriptive filename that matches the naming convention of existing images

Example: if the article has ![](images/architecture-overview.png), save to the same images/ directory.

Priority 2: Project Image Directory

If no specific article context but working within a project:

Look for existing image directories: images/, assets/, static/, img/, figures/
Save in the most appropriate existing directory
If none exists, create an images/ directory at the project root or under the relevant content directory

Priority 3: Fallback

If no clear project context:

Save to the current working directory
Use a descriptive filename: concept-name-illustration.png

Filename Convention

Use lowercase with hyphens: vector-search-architecture.png
Be descriptive: prefer rag-pipeline-overview.png over image1.png
Match existing naming patterns in the project if any

Step 5: Verify the Result

After generating:

Read the image file to visually verify it matches the user's request
If the result is not satisfactory, refine the prompt and regenerate
If the image will be inserted into a markdown file, suggest the markdown syntax: ![alt text](relative/path/to/image.png)

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