Paint should be a basic feature on any computer. It should be under Accessories. If not, search for the program itself.
http://www.notebookreview.com/assets/19101.jpg. This may or may not be what your keyboard looks like but it should give you a decent idea. Next to the F12 key is a button that will say something along the lines of "Prt Sc" or Prnt Scrn" and have alternate text that will say "Sys Rq". His this button and you should get your desktop copied to your clipboard. Then go to Paint, or any image manipulation program, and paste it. Save, upload. Should be able to do it regardless of your computer.
I don't have a desktop.
Also, I use Google Chrome and have had very little issues with it. Firefox, on the other hand, takes too long to load and slows down my computer by an enormous amount.