imgtoppm: 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/imgtoppm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/imgtoppm - from package: netpbm

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

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