Each of our print providers has different file size requirements and specifications.
Typically, you should be safe with a 300 DPI .png file.
If your image has sufficient DPI and you are still seeing a low quality error message, then you may need to change the total image/canvas size in your graphics editor to be larger.
If your image has low resolution, you may need to go back into photoshop or use other tools available to edit the image. More information in our article here.