Mike,
If you or your dad or anyone else with the rights to your dad's photos wants to modify them, go ahead. When someone else uses them, changed or not, and then sells them or uses them to their advantage somehow... what then? One of the unfortunate things about the information age is that we think we can copy stuff and incorporate it in other projects. You can't "steal" what others have written either. There's a big word for that, but I can't spell it. I suppose the problem only got widespread when the Xerox came along and then turned into a plague with the aid of the computer and internet.
Sadly, everyone does it thinking no harm is done. But just because people exceed the speed limit, run red lights or tailgate doesn't mean I should do it if I know it is wrong, or make it less illegal. Jeez, I sound like my Mom.
Flipping the coin, I do a lot of kitbashing. Suppose Bachmann is pee-o'd I changed their stuff and resold it? Doubt it, but who knows. Out.