It is an unusuall one, even a quick search on google found nothing.
You could try copying the files off as they are then renaming them from picture.ppxl to picture.jpg or jpeg (or any other photo file extension format). I don't know if this will work but it is worth a shot.
Below is a link with an extensive list of image file extensions, unfortunately ppxl isn't listed.
Now I have solved the missing file.
I have snce yesterday recieved a new information,which says, that the filetype is: ppsx. e. a. Power Point Open XKl Slide Show.