I've only used photoshop and illustrator, but if your artist uses photoshop he might want to make sure the image resolution is at least 300 (sometimes 600 depending on the screen printer) and the print size is the size you want the actual graphic to be on the tee shirt. My graphics, for instance, are usually only 4 or 5 inches. If your graphics are done in illustrator, resolution doesn't really matter since it's vector. I did a lot of trial and error, looking at the size of other tee shirt graphics and printing out my own and comparing.
That's is what the market is nowadays. Randomness. A lot of the PacSun brand names put a logo on the side about at the elbow area. Some logos look good staright in the center others have that randomness.
It depends on your design. If you are selling creative t-shirts like threadless, maybe you should vary the placement. If your t-shirts are being sold based on slogans ideas represented in the shirt, the placement should be in the center of the shirt.
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