I think that here on DSP we have people from mainly three fields: image/video processing, audio processing and digital communications. These fields sometimes feel nearly orthogonal: most of the time I have no clue what the image processing people are talking about and I mostly ignore these questions.
However, that's no reason to have a sperate site, in my opinion. The beforementioned fields all share the same tool, which is digital signal processing. It's no problem if there are "shadow communities" inside DSP. On StackOverflow there are probably thousands of expert groups which cannot contribute to the others, still they have programming in common. Tags are a tool to manage these groups.
If there must be a seperate Image processing site, it should be sharply seperated from DSP, i.e. only very specific theoretical/mathematical problems should be discussed there. Everything slightly implementation/algorithm related should go here to DSP. I have the feeeling, however, that this separation would be hard to implement. Thus, personally, I'm against having a seperate image processing site.