ppmtoicr: 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/ppmtoicr - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtoicr - from package: netpbm

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

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