Thanks to recent (last few months) update to steam there is now a function to take screenshots in game and upload/save them:
http://store.steampowered.com/news/5047/ (Note: Screenshots will be saved into your steam folder, there will be an option to see it and you can grab the shots from there).
After you got your shots you can use any image program if you want to "spice" them up a little (Photshop, GIMP, etc).
Next upload them to a image hosting site. I use
http://www.imgur.com (no ad's, popup's, cookies saved onto your PC) since it's about the best one on the 'net.
Grab the link with the
[img] tags and put it on your post. Done.
