feat(openai-image-gen): add model-specific parameter support

- Auto-detect model and apply appropriate defaults for size/quality
- Add --background, --output-format, and --style parameters
- Enforce dall-e-3 count=1 limitation with automatic adjustment
- Omit quality parameter for dall-e-2 (not supported)
- Document model-specific parameters and supported values

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/openai-image-gen/SKILL.md

@@ -19,11 +19,51 @@ open ~/Projects/tmp/openai-image-gen-*/index.html  # if ~/Projects/tmp exists; e
 Useful flags:
 
 ```bash
+# GPT image models with various options
 python3 {baseDir}/scripts/gen.py --count 16 --model gpt-image-1
 python3 {baseDir}/scripts/gen.py --prompt "ultra-detailed studio photo of a lobster astronaut" --count 4
 python3 {baseDir}/scripts/gen.py --size 1536x1024 --quality high --out-dir ./out/images
+python3 {baseDir}/scripts/gen.py --model gpt-image-1.5 --background transparent --output-format webp
+
+# DALL-E 3 (note: count is automatically limited to 1)
+python3 {baseDir}/scripts/gen.py --model dall-e-3 --quality hd --size 1792x1024 --style vivid
+python3 {baseDir}/scripts/gen.py --model dall-e-3 --style natural --prompt "serene mountain landscape"
+
+# DALL-E 2
+python3 {baseDir}/scripts/gen.py --model dall-e-2 --size 512x512 --count 4
 ```
 
+## Model-Specific Parameters
+
+Different models support different parameter values. The script automatically selects appropriate defaults based on the model.
+
+### Size
+
+- **GPT image models** (`gpt-image-1`, `gpt-image-1-mini`, `gpt-image-1.5`): `1024x1024`, `1536x1024` (landscape), `1024x1536` (portrait), or `auto`
+  - Default: `1024x1024`
+- **dall-e-3**: `1024x1024`, `1792x1024`, or `1024x1792`
+  - Default: `1024x1024`
+- **dall-e-2**: `256x256`, `512x512`, or `1024x1024`
+  - Default: `1024x1024`
+
+### Quality
+
+- **GPT image models**: `auto`, `high`, `medium`, or `low`
+  - Default: `high`
+- **dall-e-3**: `hd` or `standard`
+  - Default: `standard`
+- **dall-e-2**: `standard` only
+  - Default: `standard`
+
+### Other Notable Differences
+
+- **dall-e-3** only supports generating 1 image at a time (`n=1`). The script automatically limits count to 1 when using this model.
+- **GPT image models** support additional parameters:
+  - `--background`: `transparent`, `opaque`, or `auto` (default)
+  - `--output-format`: `png` (default), `jpeg`, or `webp`
+  - Note: `stream` and `moderation` are available via API but not yet implemented in this script
+- **dall-e-3** has a `--style` parameter: `vivid` (hyper-real, dramatic) or `natural` (more natural looking)
+
 ## Output
 
 - `*.png` images