By default, the images, and especially photos takes a lot of space. And if you store them like this it's OK, but when you want to send them by post or through messengers they are better optimized.
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JPEG Optimizer Free (Android)
On Android you can use the JPEG Optimizer Free.
When you run the application you need to select photos and Free JPEG Optimizer will automatically optimize the code.
In the settings you can specify the compression parameters, and the result can be saved in JPEG, ZIP or PDF.
Photo Compress — Shrink Pics (ioS)
Photo Compress on the iPhone works on a similar principle. In the app, choose desired image and they are compressed according to the selected parameters.