There are a number of transform options-- fit to screen, stretch to screen, center, etc.
If your images aren't the same aspect ratio as your canvas resolution, then they might not "fill" the screen without stretching, which distorts the image. You might not need to crop them all to the same size (although that might be best/easiest) but they would have to be cropped to the same aspect ratio if you want them to fill your screen without stretching.