The button should work on the first try, but if it hasn't worked yet after a few tries, there's actually a few things that could be causing that.
If you want, try it after logging in with a different browser, then using the button. If that doesn't work, use the feedback form from the image page in question with a brief explanation, like the "Delete Image" button isn't working for me on my image page and you want it deleted.
No guarantees feedback will make the delete happen though...
