photos are not in full screen on fcpx

Hello all

I've edited a video and been told a lot of photos are not taking up the full screen in the final video - so it's like there's large black gaps on the right or left side.

I can fix this by scaling photos. But i guess i want to know how to fix/prevent this in future. My photos all have a black window (see pic) so i guess it's hard to determine whether my photo fits a full frame or not? please advise

thank you.