tgatoppm: 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/tgatoppm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/tgatoppm - from package: netpbm

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

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