@sharponlooker @andrealuck
I just asked him. He posted a screenshot with GIMP, so either his GIMP directly reads the file, or he's using some plugin to do that.
Re: the header has a wealth of info about the image, orbital parameters, footprint on Mars (I presume) etc. It also says its a PDS file. I may have been half wrong blaming ESA for the format 🙂, or completely wrong about it altogether 😬