MiniPic

With this application you can easily minimize your photos before uploading or sharing them. You also can save memory because you can store up to 5 times as much pictures as you could if they were not minimized. The conversion-rate depends on the images' properties (like variety of colors) but usually exceeds 50% though.
The original pictures are kept save even after conversion so there's no risk of loosing data.

This may come handy if you're out and have no way of backing photos up to make free memory on your device.