Purchasing or finding creative commons vector images is a strategy that many artists and I use to save time and add complexity to our shirts. I would also venture to guess that many of the t-shirts you see marketed have at least some purchased vector images. Using these images does not suggest that you forgo using creativity to create a design. The vector art is used as a medium rather than a subject. Here are some examples:
Here is a good thread regarding vector images and where to find them:
Here is where I have found some great images at a reasonable price:
