Picture Packer refers to the native graphics format of the Picture Packer graphics utility for Atari computers.
According to the ST Picture Formats document, it is "a realy [sic] brainded [sic] format".
A ST2BMP program exists that can convert these files (and many others) into BMP. Unfortunately the only place the program and it's source code was uploaded to was the Atari Forum which has been offline and "under maintenance" for quite some time. WayBack machine has an archive of the forum page but sadly the program and source was uploaded as an attachment, which required user registration. Over on this English Amiga Board thread there were folks discussing it.