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.