Ideally you need to get a nice big image with a file size of less than 500kb. If you have an image editor you should change the image size to about 1200 pixels wide, set the JPEG quality to medium, and see what size that gives you. If it's less than 500kb you're laughing, otherwise you'll need to go back and try reducing the pixel width.
Our site will automatically create a thumbnail for you.
If all else fails, email the images to us and we will adjust them for you.