It is important to properly prepare your image before ordering the prints. To ensure that everything is correct, check the requirements on a photo lab website directly. They often give the requirements for uploaded files and they might be a bit different than what we wrote here.
Few general rules to keep in mind:
1. Always send the original files. Do not compress them.
2. The files should be saved as JPEG (some photo labs accept TIF or PDF as well).
3. The resolution should be 300PPI (Pixels Per Inch) or DPI (Dots Per Inch).
4. The files should be saved in 8bit RGB.