I came to the part where there's a pianist, you must kill him and take a photo.

however, i'd just got the new freezing plasmid and was going around freezing everything. so i froze the pianist, then shattered him. i then realised i needed to take a photo of his body, but there's no body to take a photo of :L and i've saved since, so what can i do?

the only thing i can think of is start a new game?

I haven't played for a while but isn't there a load game option. The game auto saves at every level start you should be able to just reload and start from the beginning of that section, i assume you are in Fort Frolic i would try loading a past auto save.

no, i stupidly saved over the autosave, i don't even know why i did it.

i'm pretty sure starting a new game is the only way out of this mess. but i can't see why they'd give you the option of being able to freeze and shatter people at that point.

i couldn't do anything, it seems, so i started a new game.

i was officially bioshocked

Unlucky, I guess. I've done that loads with Oblivion in the past, and it's made me rage something wicked.