Well it has taken a while but finally Google Mail / Gmail includes support for images in their signature files.
I’ve always liked simple text for signatures in email, but after setting it up in my own account to see how it looked, I have to say I like it a lot more.
To use this facility, you’ll first of all have to prepare a properly sized version of your logo or image – about 100px or 200px wide is fine, and make sure you’ve reduced the size of the image so it is nice and small. Then upload the image to your website so you can use it from the Google editor. If you have your own WordPress website, use Dashboard, Media, Add New to upload your logo to your own website, and click Edit to get the File URL for your image. Copy that and have it ready to paste in to the Google editor when editing your signature.
This is the post from Google introducing the feature: Rich Text Signatures