Resize the photo to no larger than 640x480 dpi. Put the photo on a hosting site like photobucket. Open the photo on photobucket, right click on it when it displays your desired size, save the location of the photo. Come back here, and when you type your message, left-click on the icon above the message that looks like a mountian with the sun over it. Paste the URL of the photo into the blank, click OK, and you're done.
As mrblanche said, all you need to do is find a photo hosting site, ie. photobucket, flickr, imgur (You probably should use imgur though because I find that this site always sensors it...
) Another option is to upload your photo into an album that you can make on here.
Once you have your photo uploaded all you need to do is right click the image, and click "copy image url". Then paste that url into the box that pops up when you click the mountain with the sun.