ok, I can't let this go.
I'll assume you use photoshop... and if you do, you'll know what I mean...
if you take an original image/screenshot... and duplicate it...
have one at 72dpi and change one over to 300dpi to try and make it "larger" for print, it will of course be larger on your monitor but did you use your zoom tool to view it at print size? it's not going to change the size your image prints out at.. unless you stretch it in some other way as well - in which case you're back to the blur effect.
unless you're originally scanning or creating an image from scratch in some way at a larger print size, just changing a dpi setting won't fix the image size issue.
if that does not make sense to you I can try and get someone else here to post regarding this topic as well.. I just don't want folks to end up with blurred out/stretched out images on t-shirts they'd like to wear..
edit: and also, who says smacking a couple different images onto a cafepress tshirt layout would be difficult/take way too much time? when you have a volunteer standing up and saying "I'll help" that doesn't take any time for anyone other than the volunteer. What exactly is it in the process of personalization (vs. a couple extra dollars in the coder's pocket) that makes it SO very hard to do?