As per the title, which stack exchange would be most appropriate for an image processing question?
Both are computer software — well, Super User is the place to ask, definitely.
Check out our existing questions for:
If you instead want to do image processing using some programming language, Stack Overflow would be better.
1Note that IIRC Gimp is scriptable. While it could be argued this is programming, we usually also accept questions about it. See e.g. VBA in Excel questions, or shell scripting.– Daniel Beck ModApr 23, 2012 at 7:47
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.