Try a different browser or device "phone, laptop, tablet. Disable any plugins like ad blockers or script blockers.
Site should have no issues handling Jpegs. unless your editing them before hand, maybe one of the settings in its file is tripping up GD. Can also try again in like a day or two. something basic shit just breaks here and comes back.
If not fixed on its own and nothing working above then this:
growdiaries.com/contact