That's easy enough:

1) Make sure that "Full Screen Mode" is OFF. (It's in the Game menu, if you need to turn it off)
2) Whenever you want to do a capture, just hold Alt and press PrintScreen (probably PrntSrn or PrtScr on your keyboard, most likely above the Insert key).
3) Then, go into MSPaint (or some other image editor) and hit Ctrl+V to paste.
4) You can then save the image (preferably as a PNG or JPG).
5) Repeat steps 2-4 for any other screenshots you wish to take.

As to "advertising" your game, go right ahead. Just make sure it's in a new thread and you should have a download link if it's uploaded somewhere.