​​1. Image Quality:   When submitting your design, images need to be at least 300 dpi.   Also, the best quality images should be saved in either PDF, EPS, or AI

2. Contrast:    Contrast is a part of color choice. The strongest contrast is always going to be black-on-white or vice versa. And of course, bright colors on a dark background are going to be high contrast.
 

3. Complexity:  Don't get carried away with trying to be creative or original, by stacking things on top of each other, using weird angles and composition, and generally creating a chaotic mess with their design.  Too many colors can be too much also.

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4. Inversion:  Inversion is something fairly common that needs to be done, usually when printing white ink on black garments. You don't want your photo looking like an x-ray.