There aren't many to be found with an online search, so resigned myself to taking them from the CD-ROM, as I did with my avatar and signature.
It's not a straight-forward process (i.e., each character, and all of its actions, is shown from eight different angles -- you have to isolate the frames for the action and angle you want; the Diablo logo file is not animated on the CD -- it's a single static image (a .pcx file) of all 15 frames. I had to crop each frame from the image, then I reassembled them into an animated gif using PaintShopPro Animation Workshop), so it's time-consuming. At least, for me it is.
I haven't had the energy to sit through the process, again, since I made my av and sig.