I have captured still frames from my video clips to use in montages. Is it possible to export these still frames into iPhoto?

You should be able to open iPhoto and hit import, then select the frames. From which software (and which version) did you export? As I recall, iMovie used to have a bug - if you exported the still as a jpeg file, it would end up cropped. I believe you had to export a pict file to get the full frame. Or maybe I have that backwards. It's been awhile. But if you come across the bug, try exporting to another format.

I use final cut pro for editing , i know that premiere is similar. if you have either program just stop on the frame you want and choose "export frame" . I find you get a much clearer still if you export 2 frames side by side and lower the opacity on the top frame in photoshop, this way you don't see the field lines.
hope it helps!