Upload your image on Tinypic or Photobucket.
www.tinypic.com
www.photobucket.com

Then, go to your edit profile/signature page on Mara.

http://www.marapets.com/forums.php?do=signature

Put
[img]YOUR URL [/img]

Then you have to put
[img]http://images.marapets.com/avatars/invisible.gif[/img]
For the image will show up.