How can I sync iPhone photos one way?
At first you should disable Photo sync in iTunes. When you open iPhoto, you should be able to select your iPhone on the left side (assuming it's connected to your Mac). Then you can select all pictures you need and import them (you will even be asked if they will be deleted from your iPhone after the import). This way the iPhone should not sync photos like it ususally does with iTunes (when enabled).

See this Apple KB article: Copying personal photos and videos from iPhone, iPad, or iPod touch to your computer

EDIT: You have to do this manually. I don't know if it's possible to do this automatically when syncing with iTunes.