At Exclaimer, we see a lot of email signatures. We see what a lot of companies get right and what they get wrong.
DO use jpegs
Remember that this is an email so PNGs, which you would use for a webpage, will not work in the way you want them to.
Choose jpegs and gifs over PNGs for almost all email instances, adding the ‘-nosend-’ tag on all images, so they’ll appear correctly in the email.