You should be able to just drag-and-drop an image into the signature in Mail - at least I could when I tried right now.
What's less professional is that I've never been able to make Mail automatically tell Outlook what font it's using on the Mac when sending in RTF format. Your mail will use a nice Helvetica font on any Mac, but any contact with an Outlook client will just see a horrible, huge Times New Roman mess unless you manually format your mail before sending it.
Any decent mail program is probably able to attach an image as part of the signature.