There are some small differences between what you see on your computer screen and what your finished product will look like. Scanners and digital cameras create images using combinations of just three colors: Red, Green and Blue (called "RGB"). These are the colors that computers use to display images on your screen. But printing presses print full color pictures using a different set of colors: Cyan (blue), Magenta (red), Yellow and Black (called "CMYK").
At some stage your RGB file must be translated to CMYK in order to print it on a printing press. This is easily done using an image editing program like PhotoShop or Corel PhotoPaint. It's best if you do the RGB-to-CMYK conversion of your images yourself. You will have more control over the appearance of your printed piece if you convert all of the images from RGB to CMYK before sending them.