I have a signature that is used in outlook. The signature has an image at the bottom and a link that encompasses the image. On the computers you are able to click on the i
I'm not sure if my suggestion is the best practice, but the image map solution didn't work for me (iOS 6.1 on iPhone 4s).
Instead, I've just placed a transparent "a href" on top of my desired clickable image:
Also, I had to leave the original "a href" around the image itself also, because the transparent link did not work on Outlook on PC.
Hope this helps.