Is there any suggested picture size to use

I had made up a show which over all did not come out too bad for a
second attempt at doing one, but like it says in the tutorial that
there are times when a picture will not fit in the screen image,
Is there a suggested size of photo to use for this program? I am
trying to figure out a way to make sure that the whole photo is
included in the presentation if possible. I had made one up that was
fairly small as it was only 32.08 kbs on the photo size but that one
had to be cropped whereas some that were much larger showed up
fine.
I am going to try to put the photo that I am talking about into the
photo section here on the site. Sandy Davenport