ppmspread: 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/ppmspread - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmspread - from package: netpbm

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

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