-hit "post reply" button
-on that screen, you have an option to attach a file (valid file extensions are listed, size must be below 100k)
-IIRC, a detailed description is found in the (stickied) FAQ in the civ3 GD forum
btw, you could also use the file upload server (nifty for several and/or bigger pics in a post) and place a link to the pic encircled by those img tags (see mentioned FAQ).
Load file "MyPicture.jpg" or whatever up, then
place the tags w/ the link in your post:
[imgX]http..../uploads?/MyPicture.jpg[/imgX]
(just leave the "X"s away)