Blacked out Photos!

I have recently started to create a movie through iMovie and it was going well. Im using the scrapbook/memory theme, and as soon I tried to drag certain photos from a folder, the entire screen appeared black. I don't know why other photos worked and why these certain ones did not. They are jpg format, exactly like the ones that worked previously. I need help asap! Is there a specific format photos must? Or is there another way to insert my photos onto iMovie without it turning black?