its really simple once you do it once.
It depends on the web browser you are using.
If the image is already on the Internet

I.E.(the long drawn out way)
1. right click the image
2. properties
3.highlight the image code
4. copy
5. return to this site and the comment box
6. click the little yellow and grey postcard (second last icon on the top)
7. Past the image code
8. press enter
(instead of pressing the icon you can type out

Firefox / chrome / opera
1. right click image
2.copy image address
3. steps 5 onwards

i dont know about macs but id say its as simple as the three above

If the picture is not on the internet already then you may have to upload it to an image hosting site.
http://tinypic.com/
http://photobucket.com/
http://www.flickr.com/
http://imageshack.us/

all free sites and easy to set up. just select the image on your computer and upload it then use the steps above depending on your browser