Are you bringing your images in as an image sequence? If so, go into the interpret footage dialog and change it to the number you want.
AE defaults to interpreting image sequences as 30fps. But you can change the frame rate to whatever you'd like in the the clip's interpret footage settings.
Watch out for frame rate details. 30 fps is almost always 29.97 in actuality, and 60 is almost always 59.94.
If you import a single frame as an image sequence it will be 2 frames long.
If you import an image sequence as an image sequence you won't be able to see the individual frames in the timeline as being 2 frames long. However, if your composition is set to 60 fps and the image sequence is interpreted as 30 fps, when you step through the footage there will be pairs of identical frames. Change the interpretation to 60 fps and the image sequence will be half as long but each frame will be unique.