CRYPT7 backups from Whatsapp contain saved history of chat conversations but do not contain any possible attachments like pictures or multimedia files that were sent using the program. So there is really no way how you could convert CRYPT7 to JPG, or any other multimedia or picture file.
In a word, direct crypt7 to jpg conversion is not possible.
So don't expect any so-called crypt7 to jpg converter or crypt7 to jpg free online conversion tool available.