If you feel that your image is somehow sacred, I recommend that you don't post it to a critique forum.
When you publish an image to the web, you lose control of the image. For this reason, most photographers limit the resolution that they publish so that the impact of people co opting their work is minimal. Legally if you post it to a forum, and someone modifies it without attempting to take ownership or make some profit off the work, I don't think you have any recourse. Get over it or don't publish to the web.
As for whether it is polite to modify someone else's work: I think if it was posted to a photography critique forum there is an expectation that it will be critiqued, and one of the best ways to accomplish that is with a quick sample of your recommended processing change (crop, levels, color balance, cloning, compositing a mickey mouse on it).
If you are posting to a critique forum, I think you should expect that others will modify your image. You are asking for feedback and advice, so obviously you are in a learning mode. It really isn't very gracious to ask for advice and then to be upset about how people give that advice. It is your choice whether to post to a critique forum.