Fliff

With Fliff you can convert formats of image files. You can load images from the clipboard, the Photos app, the Files app or from the camera. The converted images files can be saved to file system and to the camera roll and can be copied to the clipboard or shared. You can preview them in order to check the file size and the quality, espicially when using compression. Fliff can also be called from the Share menu in other apps that provide images. Furthermore Fliff is also a photo editing extension, i.e. you can use it as a plugin in the edit mode of the Photos app. 
This app is freeware. It comes with absolutely no warranty. Use it at your own risk!