A free app for Android, by OnlineConverting.

The Image Converter app is an easy-to-use tool for converting image files. It supports a large number of formats, including the most popular ones, like JPEG, BMP, GIF, PDF, PSD, PNG, TIFF, and many others.

The application also allows you to choose between a number of available conversions, such as from RAW to JPEG, BMP to GIF, etc. In addition, you can also convert from one format to another, from, for example, JPG to VTF, or from, for example, JPEG to PNG.

This is available on the Google Play Store.