ppmshadow: 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/ppmshadow - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmshadow - from package: netpbm

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

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