pstoimg: 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/pstoimg - from package: latex2html

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

On Debian possible default full paths are:

/usr/bin/pstoimg - from package: latex2html

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

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