XnConvert was developed to be a simple, yet powerful tool, that allow you to convert images. The application can handle many file formats and provides you with numerous image processing tools that can come in handy.
It supports more than 400 file formats (jpeg, tiff, png, gif, camera raw, jpeg2000, WebP, OpenEXR, ...). You can resize, add a watermark, change contrast, brightness or gamma, apply filters and many other things.
Product page: here