i think the default for taking screenies is the printscreen button (check the keybindings list to know for sure). it stores them in the following folder:

World of Warcraft / Screenshots

As for posting them in here, you could probably use a website like flickr or google's picasa to upload and host the screenies, but i adjust/crop the images in photoshop and then upload them to my website because it's just easier for me.  Once they are uploaded to an online location (also called being "hosted"), you just need the link to the image so that you can click the "Insert/edit image" button on the forum post toolbar.  It defaults to the actual size of the image, so I take a guess as to what the width and height would be adjusted to fit in smaller windows (but it's not necessary).

I can go into more detail on any of the steps if anyone is interested.