pbmtoxbm: 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/pbmtoxbm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pbmtoxbm - from package: netpbm

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

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