Resource
This fantastic website comes thanks to Jenny Baccon as a great way to find images that already have the proper citation built right into them. Head on over to http://www.photosforclass.com/ to find a ton of great images that are ‘safe’ to use (as well as adhering to our goal of being Principled users of media).
Once you have searched for your image, hit the ‘Download’ button and grab the image. 
The only hitch is that the images tend to be quite high quality, so I draw your attention to the post from a couple months back: How to Re-Size Images Using Preview.
Shortcut
This one will require a little bit of nimble finger work, but could save you some valuable time if you get used to it.
CMD + SHIFT + 4 + Space bar will take a a screen shot of an entire window that you choose, saving you the time of trying to drag the little crosshairs in order to select the window manually.
You can also change the keyboard shortcut to anything you want by going to “System Preferences”>”Keyboard”>click on the “Shortcuts” tab>”Screen Shots”.
Here you see four options:
1. Whole screen saved to desktop
2. Whole screen saved to clipboard (easy for pasting elsewhere)
3. Clip of part of screen (cross hairs) saved to desktop
4. Clip of part of screen (cross hairs) saved to clipboard
Next to each of these is the keyboard shortcut, just click on it and then press the keys you want to use instead and it will change it.
For example instead of CMD+SHIFT+4+Spacebar, you can have COMMAND+4, or OPTION+S. Anything you like, just be careful that you don’t use a shortcut you use for something else, like COMMAND+C as you probably use it for copying text already.