The best open source alternative to XnConvert is ImageMagick. If that doesn't suit you, our users have ranked more than 100 alternatives to XnConvert and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to XnConvert are Converseen, GraphicsMagick, Switcheroo Image Manipulation and pngquant.
ImageMagick is a software suite to create, edit, and compose bitmap images. It can read, convert and write images in a variety of formats (about 100) including GIF, JPEG, JPEG-2000, PNG, PDF, PhotoCD, TIFF, and DPX.
Converseen is a free cross-platform batch image processor for Windows and Linux that allows you to convert, resize, rotate and flip an infinite number of images with a mouse click.
GraphicsMagick is the swiss army knife of image processing. Comprised of 282K physical lines (according to David A. Wheelers SLOCCount) of source code in the base package (or 952K including 3rd party libraries) it provides a robust and efficient collection of tools and libraries...
Switcheroo is designed to give you a simple, quick, and easy-to-use tool to convert and manipulate your images in whatever way you like. It is built on top of the most advanced image editing libraries, ImageMagick.
pngquant is a command-line utility and a library for lossy compression of PNG images.
Phatch is an user friendly, cross-platform Photo Batch Processor and Exif Renamer with a nice graphical user interface. Phatch handles all popular image formats and can duplicate (sub)folder hierarchies.
Image Resizer for Windows is a utility that lets you resize one or more selected image files directly from Windows Explorer by right-clicking.
With BIMP you can apply a set of manipulations to an entire group of images! It provides an easy to use interface and a lot of batch functions.
HRConvert2 is an open-source web-app that converts files without tracking users across the net or infringing on your intellectual property.
dcraw is an open-source computer program which is able to read numerous raw image formats, typically produced by high-end digital cameras. dcraw converts these images into the standard PPM and TIFF image formats.
Lenna is an app to process images. Core functionality is to compress images in a batch, resize or rename images.
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It converts the type, and has less functions. But to me that's better. I already have decent image editors like Photoshop or GIMP, so those functions of xnconvert are more like a toy.
Converseen does one thing well, it batch converts images to many different types of images.
It's also open source.