What colour profile do you use?
Our lab uses the ICC sRGB colour profile. You can get detailed explanations and specifications about this standard colour profile on http://www.color.org/sRGB.html. It is very important that you are using this mode and not CMYK if you are either reworking the files in an image-editing program or if you are using a scanner to acquire the images. If you send a CMYK image for printing, quite important and noticeable colour alterations may occur.