There's a few ways, I'll post one.
First make sure it is a .gif file, if it isn't you can open it in paint and save it as 'untitled.gif' or whatever you want to name it but be sure to add the '.gif', that's how I do it.
Then go to
http://www.imageshack.us/index.php and click 'browse'. Find your picture and click on it, the address should show up in the box on the website, click 'host' or 'ok' or whatever the box says and it should come up with some adresses to copy. Copy and paste the first one in a new post and it should work.