I don't use SIB so I'm not sure where the picture frame image is located there.
I do know that it was never in the msstyles before and so couldn't be edited in WSB.
I do know that it was never in the msstyles before and so couldn't be edited in WSB.
Well, it seems I figured it out.
The "Windows 7.msstyles" theme has the user picture frame as bitmap resources (in various resolutions) at 7013, 7014, 7015, and 7016. I just exported those using ResourceHacker and imported it into the Windows 8 msstyles theme.
So I guess I didn't need StyleBuilder after all. Oh well.