I got my test items back and they look great. The printing resolution was so good that it actually revealed some flaws in my source images. Very good quality printing.
If you are going to build a store there, be sure to hit their forum for info on images. For resolution it's good to make your images the same resolution as the printed image. I.e. if the item says "8'x10' @ 100 DPI" then you should create an 800 x 1000 pixel image.
Zoom your image so it shows up on your screen at about the printed size (e.g. 8'x10' on your screen) and if it looks crappy on the screen it's going to look crappy on the printed item. But if it looks good on the screen you will be OK.
Also be aware that portions of the image which are completely white (255,255,255) will be transparent when printed. This is great for white shirts and cups but maybe not so great for a grey shirt. I used 8-bit GIF with transparency and had the printing equipment handled the transparency properly. For my next batch I'm using 24-bit PNG with white backgrounds because I am told that will look a little better.