Yahoo Mail does not allow for an image to be placed in the signature box (custom signature). Well that is not entirely true, here is a workaround that seems to work most of the time.
- Upload your image to a web server (Flickr or something similar will do)
- Put your mouse in the middle of the image
- Right Click and choose Copy image
- Go to your Yahoo mail options Signature page
- Select Show a rich text signature
- Right click in the box
- Select Paste
- Click next to the image and hit enter
- Click the Save button
It’s just that simple! Here are screenshots to follow along with.