To post a picture in an existing thread, Click on "post reply" on the right at the bottom of the thread. Then type your response to said thread and at the bottom of the "post reply" window. Select "browse" and find the picture you want to post from your computer. However, you are limited to about 100KB per picture and you can only post 1 pic at the time. So if you want to post 2 pics, you must open a new message by repeating the above steps. You can also edit your message for a time after you post it. If the picture does not go right (sometimes it does not) simply Ctrl-C the message so you dont have to retype it, delete the post and start over.
Hope this helps. Welcome to RcGroups.