I had very good results using the Image Magick tools to convert it. You may need to specify on the command line that the source image is ILBM, but other than that, it should convert to PNG (or other formats) without issue.
The only (minor) issue I had in the past was that the aspect ratio was off, due to non-square pixels on the miggy.
There's probably an ImageMagick installer package for your Linux distro.
