Not quite sure what else to write about this.
When using the editor, simply hit F9 when you want to take a nice screenshot. It will capture the view from the active viewport, sans the HUD and put it into [yourProjectName]/Saved/Screenshots/
UE4 will also give you a little pop-up in the lower right hand corner with a clickable path straight to the shot! (Ironically, I couldn't get a screenshot of this happening!)
Need a higher resolution? Take a High-res Shot!
In each viewport, there's a little triangle in the upper-left hand corner. Click there and go down to High resolution screenshot:
A dialog will pop up that will let you set the resolution multiplier as well as selecting a region of the viewport to capture.
Again, the shot will be put into your project folder, ready to be put online.
The only drawbacks are that the screenshot is in .bmp so you'll need to convert it to something more web-friendsly (May I suggest SageThumbs!) if you're going to share it. Also, as of right now, you can't take shots of other windows like blueprint graphs, etc. only the 3D viewports.
Here's a video of all this in-aciton!