I need a free image converter (program that converts images from one format to other) for Windows that can be used via command line parameters.
I've been searching for it for a long time, but couldn't find one.
Imagemagick would appear to fit the bill:
The functionality of ImageMagick is typically utilized from the command line or you can use the features from programs written in your favorite programming language.
and it's available for Windows, Mac OSX and Unix/Lunix operating systems.
GraphicsMagick is an old fork of ImageMagick that has active development and much less bloat / dependencies.