If they are stills, they do not have a framerate; if they are showing up as having a framerate of 30fps, then they are not stills.
If they are video clips, whose content is static, then the framerate really doesn't matter, especially if you're only using one frame from each. To make sure nothing weird happens when you put each one in the composition, you could right-click on the layer (after you've put it in the composition) and choose 'Time > Freeze Frame'.
If you want to sequence the layers, with one frame per picture, drag all your stills into the composition, then at the first frame, select all the layers and press alt-] to set the out-point to the first frame. Then, with the layers still selected, go to Animation > Keyframe Assistant > Sequence Layers... and make sure the overlap options are off.
There you go - a sequence full of all your stills.