No one can help since we only know what you tell us you you told us nothing.
Like what version of iPhoto and of the OS? Which Apple TV? What setting do you have on your Apple TV (and for detailed ATV questions the ATV fourm is a better place to ask)? What do you mean by "zooming" and by "cannot get photos to stay as an enlargement? What is the screen ratio for your TV? What is the ratio for the photos?
In general I expect the resolution to be ATV settings but with zero information beyound you have a problem that you do not know how to solve no one can be sure. And if you have a 16x9 screen and either 3x2 or 4x3 photos (the two most common ratios) then they will not fill a 16x9 screen without distortion