To give another deviant a direct link to this FAQ answer, use the following code in your comment or forum post:
What file types are accepted by Prints?
We accept JPEG and PNG format images. The BMP format is supported, but use thereof is strongly discouraged due to the extreme file sizes it can produce.
We strongly recommend using maximum quality JPEG images created from the original source file to eliminate loss of quality, but provide the easiest submission and approval of your prints. Further, due to the processing and production methods of our printing partner, print images supplied in non-JPEG formats will be converted to and printed in JPEG format.
When using PNG images, please do not use alpha channels (transparency) in your print source images.
Additionally, we do not recommend using the progressive saving method for print files, as it can cause processing errors for the printers. We also do not recommend the use of transparent PNG graphics.