Your graphic must be in CMYK mode. Colors may differ in RGB mode. You can go to Image > Mode > CMYK to view the setting.
Currently, this is what we suggest when setting up your print-ready file: 0/0/0/100 of CMYK. This is unique to our printers and printing process.
Your images (any graphics that are not vector) need to be 300 DPI at full size.
While we recommend using 300 DPI images the overall document you are designing in only needs to be 125dpi. We suggest placing or dragging high resolution images into this 125dpi Document and scaling as needed.