As we explained in this article about the Live Photos (and I reiterated in my review of the iPhone 6s), whenever the feature is activated the space occupied on the device will be more or less twice that of a common photo. So, especially if you have a 16GB iPhone 6s / 6s Plus, this should be used sparingly.
Anyone who thinks that the native iOS editor allows you to actually transform a Live Photo into a regular photo is wrong. It is possible to disable the movement of the photo in the editor and make it look like a normal image, but this non-destructive edition means that you can then go back to the editor and enable Live Photo again. In other words, the space occupied does not change.
But obviously, there’s an app for that.

How Lean, you can actually take a Live Photo that hasn’t looked great and turn it into a common image while retaining all of its original metadata. Using it is very simple and you can even convert multiple photos at once.
After turning your Live Photos into ordinary photos, Lean indicates how much space you've already saved on the device. That is, with it you can always leave the feature activated on the iPhone 6s and later keep as Live Photos only those that are legal.
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Recently, you should remember the tip we gave from the Live GIF app that turns Live Photos into animated GIFs or videos. Because Tiny Whale, the same developer of Lean, also has an app that does this:

Here's the tip.
(via MacStories)