The majority of browsers and monitors currently only support sRGB as a colour space. So any images uploaded that have Adobe RGB or Pro Photo profiles embedded may not appear exactly as they would if you soft-proofed an image or opened it in a dedicated image viewer.
In particular, this will be exaggerated with pro photo and your images may appear washed out and images uploaded in CMYK mode will not appear correctly.
Our image preview is intended to give an indication of scale and an accurate view of any cropping that will occur with your image. If you want to get a good indication of how the colours will print in your image you should follow our guide on soft-proofing or do a test print before you go ahead with a larger batch of prints.