Since you’re the copyright holder of the image you can decide if and how others can publish your image. Some suggestions:
If you only allow uploads in this thread, you can write something like ‘only uploads in this thread’
If you don’t mind uploads elsewhere then a creative commons license could fit, take a look at this:
I personally use cc-by-nc which allows edits and uploads anywhere as long as my name is mentioned and it is non-commercial.