Web hosting site (imageshack.com etc).
Use pic icon in top right of this "quick reply", use "select from url", paste link from hosting site and uncheck "retrieve remote file".
Use any image editor (mac or otherwise) to resize the image to 800x600 (or less). When you save the image it has to be 150K or less.
When I save on a PC using Paint.net I can adjust the quality; while doing so it shows the estimate file size. When I get just below 150K I know I'll be able to upload to the forum.
Do you have iPhoto? I seem to recall that being on my MacBook Pro years ago.