Photos taken with your iphone are not part of the sync process at all.
You should be importing these pics to your computer regularly as you would with any digital camera.
"So when I "back up" my phone, it doesn't store them either?"
The pics are part of the backup. They cannot be viewed on your comptuer, they are only saved with the settings, and are only intended to be used to restore your iphone. This is not a an effective way to bckup your pics. You risk losing them.
You should be importing all pics taken with the iphone to your computer regularly. Camera Roll on your iphone is temporary storage for pics taken with the iphone.
"If my phone were to crash right now, the back up I did yesterday wouldn't save my photos?"
Maybe. You may be able to restore them to your iphone, if nothing has gone wrong. Do not rely on this to backup your pics.