As per the title, which stack exchange would be most appropriate for an image processing question?
There's also a Graphic Design Stack Exchange:
Graphic Design is for questions targeted toward 2D design professionals, students, and enthusiasts.
This includes questions about:
- Graphic arts theory and history; "Why" we do what we do and not necessarily the "how" in logo design, fonts & typography, visual communication
- Web design
- Layout and printing
- Digital media
Examples: video, Adobe Flash
but not about:
- Simple "How to" questions
- Brainstorming or idea gathering
- Technical questions related to 3D software
- Technical questions related to video editing software
- Web site architecture and development
Examples: HTML that doesn't include CSS, server-side issues
- Landscaping and architecture
If what you want to do is post-processing of photos, I'd suggest Photography Stack Exchange.