An alternate method is to use the Grab utility, which comes standard with OS X. To run this application, go to the Finder icon at the upper right-hand corner of the screen. Type in "grab". It should be the top hit and is accompanied by an icon, the most noticeable feature of which is a drawing of a scissor. Click on it (later on, you can make a shortcut to it and place it on your Dock, if you find yourself using it frequently).
I just now discovered that you can set it to automatically save screenshots in the format of your choice (one of several hidden options, which requires you to run the Terminal and type in a command line. This is easy, just follow this link:
Do not be afraid of the Terminal application and entering in command lines. Used carefuly, they can provide many benefits! http://osxdaily.com is a website which often features helpful tricks that are accomplished via the Terminal application. Trying some of these is a safe way to familiarize yourself with Terminal.