If you're a windows user, you can simply open the source image in your favorite image viewer, and using GhostWin[1], make Photoshop transparent so that the source image is visible beneath your canvas.
On Mac, Afloat[2] seems to be able to do something similar, but I've never used it.
[1] http://www.ghostwindows.com/
[2] http://infinite-labs.net/afloat/