XnConvert is an easy image converter for graphic files, photos and images. It supports more than 400 file formats (JPEG, TIFF, PNG, GIF, camera RAW, JPEG2000, WebP, OpenEXR, etc.). You can resize, add a watermark, change contrast, brightness or gamma, apply filters, and many other things.
Note: While the software is classified as free, it is actually donationware. Please consider making a donation to help support development.