ppmdither: 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/ppmdither - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmdither - from package: netpbm

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

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