Powerful and free
XnConvert is a fast, powerful and free image converter for Windows, MacOS or Linux or even Android and iOS. Convert your image files to over 500 different formats like JPEG, TIFF, PNG, GIF and WEBP. Additionally, edit, rotate or compress your images, photos and pictures easily. The software enables you to use over 80 different actions like resizing, cropping or adjusting the colors. You can save and re-use your presets for another batch image conversion.
The converter is free, without restrictions or adware. All they ask for is a donation if you enjoy their services. However, companies need to register a license.