ppmtowinicon: 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/ppmtowinicon - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtowinicon - from package: netpbm

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

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