You must use an image hosting site, like photobucket.com or imageshack.us and upload your pictures there.
After uploading, copy the picture's URL (ends in ".JPEG"), come back here, and click on the photo button above and paste it.
The photo button basically adds the code [IMG][/IMG] around your picture URL for it to show up on here.
You can also manually type [img][/img] around the image URL for it to show.