Use a good quality camera of at least 8 megapixels.
Adjust the picture size or quality settings to take pictures of the largest possible size or highest possible picture quality.
Take close-up pictures against a plain (single colour) background.
Check that your picture has a good sharp focus.
Check the dimensions of your image files: they should be at least 2400 pixels on each side.
Finally, please send your original jpeg images. Don't convert them into eg Word or pdf as image quality will be lost.
Note: If the images are very large they will take a while to upload.