Is it possible that your phone could be filled up with photos?
Photos are stored on the SD card. Apps are on the internal memory.
If he's getting a warning about internal memory being full, it won't be pictures, videos, music, documents or anything like that. All those are on the SD card.
Regarding the original post:
1) Some apps are system apps. You won't be able to uninstall those unless you are rooted, so if you see something you can't uninstall, that's why.
2) When you go to manage applications screen, you can sort the applications by size. That will help you see what is taking up the most space.
3) You may not need to uninstall apps that are using a lot of space. Some apps cache a lot of data, and when you go into the app, you can clear cache and clear data (instead of uninstalling). Note that when you do this, accounts and settings you setup in the app will all be reset, and you'll need to setup those again. However, this can clear out some old stuff and when you setup again it may use less space. Try this before you uninstall an app you actually use.
Beyond that, look for any apps you tried once but haven't used. Get rid of those.
The suggestion to move some apps to the SD card can also free some space.