Another cross platform image conversion tool is imagemagick. It is strictly command line driven, but very powerful. Imagemagick is a suite of tools that manipulate images. The most used commands are "convert" and "mogrify". Converting lots of files at once can be done with a single command, e.g.
Code:
mogrify -format tif *.png
converts all the .png files into .tif files. It handles an enormous number of file formats and transformations.