DTG - Direct to Garment printing. A special type of printer physically prints material to your items
Dye Sublimation - A special type of printer prints special ink to a special paper. you then heat transfer the picture to your shirt (this is NOT the old fashioned heat transfer type)
For good information on this topic go to the t shirt forums and do a LOT of reading.
Free dog clothes would be the fur your dog was born with - a dog doesn't need clothes (unless it is hairless or elderly in which case buy it a dog coat for going outdoors)
The only free dog clothes i would recommend if you own a poorly dog that needs protection from the cold is to cut the sleeve off an old sweater, making the cuff the head end cut two holes for the front paws to go through and hey presto FREE.
Do yourself and your dog a favour if you are planning on dressing it up in some ridiculous outfit for the sake of fashion DON'T.
A dog isn't a fashion accessory to be carried around and dressed up - thats what dolls are for and just think how silly you'd look carrying a doll around (thats how others see you if you dress up a dog!)