pnmflip: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/pnmflip - from package: netpbm-progs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/pnmflip - from package: netpbm

You may install the required package with command apt-get install {package}

Error in other languages:
pnmflip: bevel nie gevind nie
pnmflip: comandă negăsită
pnmflip: comando não encontrado
pnmflip: commande introuvable
pnmflip: command not found
pnmflip: käsku ei ole
pnmflip: không tìm thấy lệnh
pnmflip: komanda nerasta
pnmflip: Kommando nicht gefunden.
pnmflip: kommandot finns inte
pnmflip: komut yok
pnmflip: nie znaleziono polecenia
pnmflip: níor aimsíodh an t-ordú
pnmflip: no se encontró la orden
pnmflip: no s'ha trobat l'ordre
pnmflip: opdracht niet gevonden
pnmflip: parancs nem található
pnmflip: perintah tidak ditemukan
pnmflip: príkaz nenájdený
pnmflip: příkaz nenalezen
pnmflip: команда не найдена
pnmflip: командата не е открита
pnmflip: 命令找不到
pnmflip: コマンドが見つかりません