XnConvert is a cross-platform batch image-converter and resizer with a powerful and ease of use experience. All common picture and graphics formats are supported (i.e. JPG, PNG, TIFF, GIF, Camera RAW, JPEG2000, WebP, OpenEXR) as well as supporting over 500 other image formats.
Also available within the batch operations include rotating, adding of watermarks, adding of text along with many image-adjustment features such as brightness, shadows and more.
XnConvert is free for Windows, Mac, and Linux. Both 32-bit and 64-bit editions are available.