There are two options to post a picture-
The first is using an image hosting website such as imageshack.us or photobucket.com. The latter requires you to register for a free account, but by doing so you can keep track of the pictures that you upload. When you use imageshack.us, click on browse, find the picture and click upload. Once it uploads use scroll down and copy and paste the Hotlink 1 URL and paste it into the message box here. On photobucket, it's the same except you should copy and paste the IMG Code URL.
The second option is to upload through this site. On the main reply screen, scroll down a little to the Additional Options section and click on Manage Attachments. From there you just browse and upload and that's it. No copy and paste necessary.
Hope that helps.