Find an image hosting site that you like to upload your images to. Copy IMG or URL code.
My Stuff > Edit Profile
Scroll down and paste your URL or IMG code in this area....
Click preview to see if it worked.
Some rules regarding signatures...
15. Signature images may not be larger than 600 pixels wide by 125 pixels tall. This includes logos, photos and other graphics. Also, no more than one row of images are allowed. If you want four or five logos in your signature, reduce their size to fit in one row.
16. Limit the size of your photos to a maximum of 800 pixels wide. (this does not include signature images - see rule #15 above)
Use his link to resize it if you need to -
http://www.simpleimageresizer.com/