For a test, try the following... (First, back up ALL your save games!!!
) Mount the image, launch the game, and get into a situation where something is weird (high prices, cannot buy, cannot sell etc.). Save the game, exit the game, unmount the image, insert the original CD/DVD, launch the game, reload the game and see if you can see a difference.
If you can - things get back to normal -, then this really must be the copy protection. If you see no difference then it's either a bug in the game (or an intentional decision from the developers?!) OR, perhaps, the fact that you once used an image is already stored in the saved game ("eternal punishment"
).