There are two ways of looking at it. One is the purists way that many here are advocating, the other is the somewhat less puritanical but more practical way.
Bear in mind that I am a professional photographer that spent two years and many, many thousands of dollars chasing a firm that illegally used my images to build commercial websites, so I can certainly see it from the other side too. Incidentally, I recovered my costs but that was it.
I advocated using Vimeo simply because they do not yet have the restrictions that YouTube does and I feel that to use an artistic work, be it a photograph or music or whatever, for non profit personal use, is an acceptable use. If someone wants to use one of my images as a screensaver, good on 'em.