You can't remove anything once you've posted it, so it's best to make absolutely sure you've wrote everything right before you post it
We don't remove anything unless it in some way violate the site rules either so the best you can do is checking the little box making you anonymous then people won't be able to track the post to your account. ^.~
I'm not familiar with the deletion of the pictures from their original site solution, that some here are mentioning, but if that works, that's probably your best bet. ^.~
Once it is on the internet it is there forever if I were you I would contact GAG and see if they can delete your account and then you can start a new one but if anyone were to screen shot the pic it could end up anywhere.
Umm it depends. There was a picture I had in a question of mine that I wanted to delete so I just went to photobucket (that was where I uploaded it from), I signed into my account, deleted the picture from there and it didn't show up on my question anymore. So if you uploaded it from somewhere like photobucket, then you can try it that way. Other than that, I have no idea, sorry :p