not trying to hate, but do you want to look like a douchebag in youre senior pics? wearing a shirt and a tie, aswell as a polo for 2 pictures isnt preppy, nor a big deal. im g'd out from teh feet out all winter, but in teh summer i rock polos all day errryday. for something like this by not wearing something like a poloyoull look like a tool.
if you really insist on not wearing a polo, as it is your choice, id go with none of those, and either a blank tee that fits, or a nice crewneck/sweater