You can't change an image on a deal after it is posted. The best thing to do is post a comment on the deal letting a moderator know that you want them to change an image. Sometimes a moderator will fix a deal that has no image without you having to ask.
Being able to change an image would be a great feature. I was going to post a freebie & found it already posted without an image. I think that person's deal would have had more plusses if there was an image.
I agree with glwrks ,one picture is worth of thousand words.This happened with me too ,i could not upload image.you can not do anything after posting a deal but before submiting you can do google and upload similar image.