I need to convert tons of pictures, coming from all kinds of sources, in all kind of file formats. I want to store them all in the PNG file format (even if it means losing e.g. layer data from ESP's).
I'm looking for a library that is preferably open source and has a Java or Groovy API (on top of JNI/JNA, if necessary).
ImageMagick is a very potent image manipulation library and there's a JNI wrapper available.
Otherwise the JAI library supports reading and writing .jpg, .gif, .png, .tiff, .bmp and other files so you might try that.
It depends mainly on the range of formats you need to support.
If you want to stay in Java you can use the ImageIO and image related classes of JDK java tutorial adding may be the Java Advanced Imaging link text
But these will support basic formats like:
Maybe additional image writers are available to be plugged in into this structure..