Resolution should be set to 300 dpi.
Pictures and graphics pulled from the internet are often low resolution, typically 72 dpi or 96 dpi. Avoid these graphics, as they will appear pixilated and blocky when printed.
Also note that you should save all photos in CMYK mode, not RGB mode when possible. Images saved in RGB mode may not print properly. If you are unable to save your image in CYMK mode, please let us know.