XpresS, are you using a Mac?
Just curious. If you are, I believe the bindings are a bit different for taking screenshots. Not sure, though.
To check if it's even bound to a key, go to your options and click on the keyboard tab while in CS:S. Scroll down the list until you find something titled "Take Screenshot" and see what key it was assigned. If it's not assigned to one, assign it to f5. That key should be at the top of your keyboard, and it should be the sixth one in from the left. Not sure about Mac keyboards, though.
Now, go ingame, take your screenshots with that button at key points in the map that should be highlighted. Only take about 5 of these.
Upload them to
http://www.imageshack.us/ - This site will not delete your pictures unless they break the rules, which basically is nothing obscene like nudity, which would break the eGO rules anyways so you wouldn't be able to post it without getting in trouble. However, it's a pain to find them again if you accidentally lose the link, in this case, you should probably make an account on there as well if you're worried about that.
Then, copy the link to the album, or go into each picture and copy the forum thumbnail links and paste them on the forums in the original post by editing the post and right clicking the editable section on your screen and clicking "paste," but make sure it's at the bottom of the text. After you've pasted all the links in, erase your comment about not being sure about how to post the screenshots and you're good to go and can click Apply Changes/Accept Changes or w/e it has.
Hope that helps you. ^^