Popular Science recently posted an article “Your smartphone photos take up too much space. Here's how to downsize them."

Firstly, don't do it. The solution the author provides is defeatist. Photos never take up too much room because, as a general rule, the more data you have the better your photo will look. For example, 14-bit RAW files from a professional camera have insane amount of detail and information. One has so much flexibility and data to work with that editing becomes very powerful.