It took about an hour and 15 minutes for poor Jon to troubleshoot this issue remotely for me.
Google had several tutorials on how to add images to your gmail signature, but most of them referenced Firefox add-ons that no longer supported this function, like Better Gmail.
Open Firefox, go to Tools > Add-ons > Get Add-ons, type in “greasemonkey” then download the add-on and restart Firefox. Or, you can download it here.
Then go back to Tools > Add-ons > Get Add-ons and type in “wisestamp” and install. Restart Firefox. Or, you can download it here.
Go to Tools > Add-ons > Extensions. Select WiseStamp and click Preferences. This window will open. (Fields will be blank.)
If you have an HTML version of your signature, paste the HTML into the editing window. You can make adjustments to your formatting using HTML.
If you don’t have an HTML version, click on the Visual button above your editing window. This will let you make changes in the editor, which is similar to the visual mode in WordPress or even Microsoft Word.
Make sure any images you use are uploaded somewhere. If you use the HTML mode, just use regular HTML code to insert the image. If you use the Visual mode, use the insert image icon and enter the image URL.
You can also add any social services (LinkedIn, Flickr) or contact methods (IM, Skype). These will appear below the information you entered in the editing window.
If you don’t want to promote Wisestamp, go to the Settings tab and check off Hide WiseStamp statusbar icon and uncheck Promote Wisestamp Project.
Use the Preview button to test your signature, and be sure to save your work. Log into gmail and click Compose. Your signature should show up automatically.
Thanks again to @jonheller for finding the add-ons and figuring out which ones worked the best. If you use this tutorial, please tweet him and tell him he’s great!
Good luck with your signatures and please feel free to post any questions.