Doesn't MAC save screenshots as PICT files? If so, you can open that in any graphic editor, do that then save it as a gif, png, or most easily JPG file and then save it to http://www.imageshack.ws
and copy the link in image tags here.
Also try this prog- http://www.ambrosiasw.com/utilities/snapzprox/
(there's a demo version on there), it looks pretty good.
Hold down Control + Command + Shift + 3 or 4. This pastes whatever you choose onto the clipboard, and then you can just open up Preview, go to New from Clipboard and save it as jpg or whatever other format you like.
Hope that helps...