Go to photobucket.com and set yourself up. The upload pictures to your account then all you need to do is post the URL of that picture.
Once you have opened and posted a picture to photobucket, what I have said above will all become very clear. It lists the URL and HTTP of that picture underneath the picture.
Your signature then asks for a URL - copy and paste and you are away.
Also if you want to put a picky in a post same thing. click on IMG copy and paste the IMG details from under you picture in photobucket and hey presto.
Good luck. Post a pic on this thread so we can see you have managed it.