There's a good thread on the subject of photo resizing and compressing in the Administration Announcements Forum (top one). Look for a thread started by Rudy Taube near the bottom of page 2. It is largely PC oriented, and I admit to have tried to help a friend with pictures and his MAC, but I didn't understand his computer at all, a problem.
You'll get myriad suggestions for software to use. I had some stuff on my computer when I got it, and both Olympus and Canon software, and couldn't figure out any of them. My brother suggested IrfanView, and I found it easy to figure out with no problem. There is a version of IrfanView for MACs, and it's freeware (no charge, but donations are acceptable).
A) Start out by giving your picture a descriptive title, and 86 all the OEM crap that means nothing to you. My system is to add a % for each rework level. I also suggest using that folder on your desktop (if MACs have desktops and folders).
2) Basically, you want to reduce the pixel width to 600 > 800 pixels and keep the aspect ratio the same. Pixels are the little squares that make up pictures on TV's, LCG depth finders and Digital Cameras of all types. Too wide a picture on a forum usually makes the text of the message (and all following posts) that wide, requiring the reader to scroll back and forth to read each sentence...a PITA.
III) And then, you want to "compress", which is usually some sort of % rate or 0 > 100 scale. You do this just as you save the reduced version of the picture to your computer. It should all work somewhat the same, MAC or PC, so refer to the thread on the Administration Announcement Forum for more details...the light will come on. Maybe you know somebody who can talk MAC to you on the phone?

Steve