try enabling the Manually manage songs and videos box in iTunes. you can manually manage your music from one computer and have your photos from the other computer sync to your iPhone. i havent tried it personally, but i think that should work.
it's very possible. I know because I do it. I sync media(videos, music, ringtones) from a pc and sync contacts, photos, calendar from my mac.
You just gotta connect your iphone, open up itunes and select what you want to sync.
Please note, when syncing music, videos and ringtones, it must be from the same machine(pc or mac) I tried syncing just music from my pc and videos/ringtones from my mac and I was unable to do so.
but for your situation, you should be able to sync photos from your mac and music from your pc just fine.