ppmfade: 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/ppmfade - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmfade - from package: netpbm

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

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