A yellow exclamation mark (!) or a red cross (X) appears on my photo on the screen. Why is that?

The yellow exclamation mark and red cross are warnings that the image that you have used for your photo are of a low resolution, which may give poor results when printed. We strongly suggest that you do not proceed with your order if these warnings appear. To continue with your order, either reduce the size of the image until the warning disappears or try uploading a larger size image.