go to a picture hosting site like tinypic.com / photobucket.com
locate the picture and upload it.
copy the IMG code and paste it
I suggest tinypic. You don't need to register, but your pictures are viewable to all.. I think?
I also like photobucket because it's more official and you can have an account and hide the photos, plus I think the quality comes out better sometimes.. plus you can edit the pictures right from photobucket.