I'll save you the effort of the search.
Past the bottom of the box, where you are entering text for the topic, click on the "+" in front of the words "Additional Options". You'll see Attach...
Hope that's enough of an explanation.
If you have trouble posting photos because they're too large, you can use this free download to resample, resize, or reduce quality to reduce the file size so they'll post.
http://www.irfanview.com/main_download_engl.htm
