wkhtmltoimage: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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 Debian possible default full paths are:

/usr/bin/wkhtmltoimage - from package: wkhtmltopdf

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