The folk I use for t-shirts will do a small design on the front left tit, and a large design on the back of the shirt, for less than a tenner a shirt (initial uploading fee per design 3.49) The design can be different front and rear. They only do white (pish), grey (somewhat gay, generally) and black (the proper colour [with a "U"] for a t-shirt) and sizes up to XXL (iirc).
A square image of 3000 x 3000 pixels is ideal, as it will be reduced to fit either design area (small front, large rear) and I do believe they can do photographs, so details shouldn't be lost.
Apart from the "iron-on" feel and look of the printed area, which soon goes away after a few washes, I think the t-shirts can look good, so long as the image background is black, to suit the t-shirt fabric.
I'm having a couple of new "yeti-monster" ones printed shortly (with the new "eclipse" logo) and I'll probably have a "Hardley- Ferguson" one done too.
If there is enough interest, I may well have some made up for the impromtu bash as well.