Why does anyone need such a thing? The mac has built in functions to take screen shots.

If you want a shot of a webpage complete open your browser to that page.
TYPE command+shift+4 keys then once you have the cross hair all you need to do is press the spacebar to switch to item mode... this will show you a camera icon and anything you hover over will be highlighted to show that is what you will be taking a shot of. click your browser window and you have a shot of the page. if you don't press spacebar you can simply use the cross hair to select anything on the screen. Or if you want the entire screen command+shift+3.

Learn to use what is built in to the System without adding useless junk