It lets me....But I haven't been boo'ed so I won't put one on for good.
Ok. Go to "edit your signature"
Click on the icon that looks like a photo of a mountain.
In another window, go to the web site (not saved on your computer) that you have your boo'ed bouncy Mickey saved. There you will hopefully have options such as copying the url or the img, or???
Copy the img
Go back to the screen where you are editing your signature and paste the img in the box that opened when you clicked on the icon. Caution - you may duplicate the img at the front and back of your paste. If you have 2 at the front and back of the paste, delete one of each..
Click ok, click save or whatever..Should work.