Well you either need to upload it to your profile here or somewhere else on the web. Then right clic, go to properties and copy the file name completely from http to the end which is probably jpeg. After you type in your text in the reply window, space down a few and then clic on the window frame in the tool bar, a window pops up, paste the file name in and submit.
If its the size you want fine, if not grab the edges to stretch and post