For a newsletter or for personal email?
For a newsletter, you should do both versions. Most newsletter sending software has this feature built-in.
If for personal email, I tend to recommend that textual information should stay textual and graphics should be limited to a company logo. Not all mail clients support HTML email, and if these clients do not, your email will automatically be reduced to plain text. If all your signature information is in an image, that image will be discarded in text-only mail clients.