You'd need to replace one of the dimensional symbols for that; or else if the image completely covered the symbol you could just add an onPaint handler. One way to change the symbol would be to patch the key that defines the image to point to another image resource. I'm not 100% sure that the monster images are read from private settings, but I think so. Still, you might need to save and reopen the modified token. There are examples that do these sorts of tricks in the plug-in authoring kit.
Adding a dimensional symbol to the front of an investigator card might be easy, depending on where you want to put it. For example, you could easily insert it at the bottom of the special abilities table by adding an image tag. Otherwise, you probably want an onPaint handler again.
Cheers,
Chris