the best way to totally customize a design is to draw it yourself, then screenprint it yourself with some old panythose, a frame, and some screenprinting ink. here’s a guide that i used and it worked really well for me: http://www.threadbanger.com/post/490/diy-screen-printing-how-to
note: use screenprinting ink instead of "fabric ink" like they suggest, it will make the ink go on more smoothly and prevent the non-dyed parts you can see in their final product.

if you’re adventurous you can try painting the shirt. in my experience acrylic paint dabbed on very lightly with the back-end of a paintbrush over and over comes out very nicely. it also allows you a lot more freedom with lines and such. looks a little more diy, but personally i like the effect.

then of course you can send it to cafepress.com, you upload the picture and they put it on a shirt for you. i’ve done this and the quality isn’t that great, and you have to make sure there’s no pure white in your design or it will come out the same color as the background. basically these shirts look like something from a kinda cheap department store.

hope this helps!
References :
http://www.threadbanger.com/post/490/diy-screen-printing-how-to