PhotoShop aside, what about a negative-image color inversion? That might make the shape/size of the pop rivets...um...pop more.
Symmetrical rivets seems more like a natural conclusion, but we can't yet rule out asymmetry. Sure, they put the rivets on the non-camera side of the prop, but they probably would have wanted the look to be as clean as they could, just in case. Sort of like how they painted the ESB grip screws black in an attempt to hide them.
Correct me if I'm wrong, but the two rivet sizes would require two different drill bit sizes to bore into the flash tube, right? It would be more work to swap out drill bits than it would be to just grab four rivets of the same size. I can't imagine a dedicated prop crew having a rivet shortage.
Also, just to float the possibility out there: Maybe each set of rivets was added at different times, when it was decided that extra reinforcement was needed. Although the specific positioning of the rivets indicates it was all planned out at once, the rivets added, and the grips laid in after.
Of course, there are those shots (Sandcrawler scene) which feature a missing grip. Maybe the prop had a little accident, early on, and needed some repairs, like adding the rivets and reattaching the grip. Just spitballing, here.