dvipng: 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/dvipng - from package: dvipng

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

On Debian possible default full paths are:

/usr/bin/dvipng - from package: dvipng

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

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