We suggest the industry standard resolution of 300 DPI for optimum quality printing. A minimum of 240 DPI is required. sRGB color space is required. Images tagged in other than sRGB color will yield less than optimal results. 
For Print-to-Bind orders, our system will not accept files that are anything other than sRGB. If you receive an error due to color space, please contact customer service and we can tell you exactly which files are not sRGB.