comicthumb: 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 Debian possible default full paths are:

/usr/bin/comicthumb - from package: comix

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

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