Not everyone knows this little trick for all Android devices with Ice Cream Sandwich (4.0) or above. To take a screen shot, all you have to do is press the Volume Down button at the same time as the Power button, and you will hear a camera shutter sound and see an animation of your screen turning into what looks like a photo. The new screen shot is placed into your gallery, where you can do what you will with it, such as moving it to your DropBox.
It took a few tries to get the timing right on my phone and tablet. As one can imagine, all the first few attempts did was either power off my device or turn the volume down. Be warned, the timing has to be precise. But this is a good trick. No need to download an app!