xpmtoppm: 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/xpmtoppm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/xpmtoppm - from package: netpbm

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

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