Doing a screen capture on the computer is as easy as Using the print screen key on your PC or "command+shift+4" on your Mac. if you want to draw or annotate on that screen capture try using Snaggy, a web-based tool for drawing on, annotating, and sharing screen captures.
To draw or
write on your screen capture just paste your screen capture image into
Snaggy. Snaggy offers tools for highlighting a section of your screen
capture, typing on it, and drawing free-hand on your image. You can also
use Snaggy to crop your image.
When you're ready to share your screen
capture, Snaggy assigns is a custom url that you can Tweet, email, or
post anywhere you like.
I added the word Picasso and the red arrow
Snaggy lets you save your edited screen captures
to your computer as well. I saved my image to my computer by opening the provided link and then copying the image, opening PhotoShop and then pasting it into a new document and saving as a jpeg...this is a few steps but I could figure out any other way to get it to open. Then the image could be used in Photoshop, Keynote, or in other applications.