There are a number of resources to insert html signatures in Gmail. But, since I want more than one to handle multiple email addresses I'm sending to Gmail, it was difficult to find one that would allow me to choose a signature based on which entity I was emailing as. Well, there's a solution using bookmarklets.
Go to this site, and you can set up multiple html email signatures as bookmarklets. Then you just drag the bookmarklet to the brower toolbar, and you have what you need. I have four signatures done this way, and just click on the right one when I'm in the body of the Gmail.
One little weird thing, though. The checkbox to "Insert signature above reply," seems to do just the opposite. It could just be that I'm thinking backwards though. Try it first without that box checked, as that's the way I found worked best for me.
Ah… I was looking for something like this… it generates the html for you, so be sure to upload the picture or logo you want to your own server and take a note of the URL of that image file as you will need to specify it. Thanks!