The simplest way to substitute graphics in a different format is as follows:
Then you can simply map the old extension to the new extension, as follows:
[Graphics] FileNames=Map FilePaths=None
[GraphFiles] oldext=newext
Do not include a leading dot when you map extensions. For example:
[GraphFiles] jpg=bmp
However, if some of your replacement graphics have different base names or are in other directories, mapping old to new files becomes more complex. See §40.2.2.1.2 Substituting files with different names or locations.