Can somebody tell me if I can access my PC directly on appleTV - or is it only a device designed for the iTunes music industry?
No - current AppleTVs either stream media from the internet, local iTunes libraries on the LAN(Win or Mac) or via Airplay/Airplay mirroring from iOS devices (iPhone/iPad/iPod Tocuh).
Is AirPlay something you can use for direct access to your PC?
No AirPlay is for iOS and recent Mac only to send visuals to AppleTV.
You may be able to share photots under:
iTunes>File>Home Sharing>Choose Photos to Share
this is for iTunes 11 on a Mac, it may be the same on PC.
Is it possible, convenient or necessary to make a "jailbreak" on appleTV to use it without iTunes complications?
Jailbreaking cannot be discussed here, even if it were possible.
Don't go there - if AppleTV doesn't do the job take it back and get something else that does.