Hi, Catherine-
I am going to *guess* you are running on a Mac? If so, to install netpbm, you can follow the instructions here:
[
macappstore.org]
using the Homebrew package manager (note that the syntax for those commands is using the bash shell, so if those fail, you can type
"bash", then copy/paste them, and when done you can type "exit" in the terminal).
If you are on Ubuntu, you should be able to do:
sudo apt-get install netpbm
and be sorted.
--pt