How to revert image modifications

Needing it "soon" doesn't affect our willingness to help you because urgency on your part doesn't imply an emergency on our part.

You're going to have to setup some sort of "undo" feature so you can undo changes in the reverse order that they occurred. The brute force approach is to maintain a copy of the image after each change. Since some images can be rather huge, you'll probably have to limit the number of undo's that the user can - ummm - do.

I suppose you could xor the image so that only the pixels that changed are kept in the undo buffer, and then you can simply reverse that process to regain the previous image iteration.

You can also maintain a layer buffer that only has the changes that were made in each layer. when the user wants to undo, simply delete the newest layer from the list. When the user saves the image, merge the layers, and then save.

Unfortunately, this is not a trivial task where images are concerned, and you'll have to come up with the code yourself. On the bright side, you'll be 1 step closer to being able to call yourself a programmer, and you can post an article here describing how you accomplished your task.