There's probably a simpler way, but this is how I do it from my mac;
Open Iphoto, select your pic.
File > Export > select quality and size > Export button > Name the pic, select "desktop" > OK >
Back on the forum.... Post reply > Go advanced > Manage attachments > Add files > Select files > Select your file from the desktop list (select desktop if it doesn't come up that way automatically) > upload file. (if you get a error message the pic is too big and you need to go back to step # and make it smaller and or reduce quality) > hit submit reply