One possibility is if you are repeatedly saving the file as
by Kiddpeat - 11/20/06 7:44 PM
In Reply to: Corrupt jpg files - missing colors on half or more of image by skimmet
a jpeg. Each time you save a jpeg, some information in the photo is thrown away. This loss is cumulative. It will be especially aggravated if you save at less than maximum quality level.
Progressive damage, or corruption, while stored on a hard drive is extremely unlikely. That kind of damage would render the image unreadable. The change stems from something you are doing.