[Tuxpaint-users] Tuxpaint and printing

[<api...@t-...>]

Help,
Configured tuxpaintrc in home directory and every configuration change 
in this file seems to function correctly when changed. e.g. Sound/no 
sound  Quit/No Quit etc.
However when I try to print the program crashes or just closes. I have 
tried various configurations in the tuxpaintrc file with printing, I 
receive however constantly the same error:

sh: pngtopnm: command not found
sh: pnmtops: command not found
lpr: error - stdin is empty, so no job has been sent.

Help anyone?
Many thanks