pbmtowbmp: 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/pbmtowbmp - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pbmtowbmp - from package: netpbm

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

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