You need to make sure all the images you send us have an embedded profile. Often Adobe RGB (1998) or sRGB but depends on the image's creation or capture.
We recommend capturing or creating your image in Adobe RGB 1998, which offers a broader colour gamut and is, therefore, more likely to give you richer and more detailed prints.
It is essential that you send us a file with a tagged colour profile, as we can't guarantee accurate colour reproduction if the file supplied is not with the correct colour profile.
If you'd like more information, visit Adobe's colour management help page, or give us a call.