Photos should be in aspect ratio 4 (wide) x 6 (high)pixels and no more than 1000 Kb (1Mb) in data size. Save as .jpg files. For best results resize your image to 120 x 180 px , (The application will resize for you with the ratio of 4 to 6. so 1200 x 1800 or 600 x 900 will work)
There are a number of programs available for cropping and resizing images and many can be downloaded for Free. Paint in Windows and Preview in Macintosh will do the job.