I'm not sure how to do it with custom properties, but you can do it with drawing layers.
Add the pictures where you want them and make them part of certain layers. Turn them on/off as needed.
There may be a way to use these layers in custom properties. I have not tried it yet.
If you want this image to always be in the same place I'd suggest setting up a sheet format for each one. (That way the image's position would be locked down as long as the user didn't go into "Edit sheet format" mode.) It's pretty easy to switch sheet formats as needed, though I don't know of any way to have them linked to the model.