By saying "Microsoft 7 Paint" do you mean Windows 7 Paint? If so, once you exit from Paint whatever edits/changes you make are saved and you can't go back to edit it out.
However, if you are using Windows 7 you can use Windows Photo Gallery and use the "Retouch" tool to hide the symbol. Here is an easy to follow Youtube video to show you how it's done:
Retouching Photos with Windows Photo Gallery
Make a copy beforehand to be on the safe side
Good luck