ppmtopict: 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/ppmtopict - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtopict - from package: netpbm

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

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