Your pictures get compressed hard by the website. I've noticed a huge loss in quality on mine. There aren't any recommendations by GD themselves though.
It's definitely more cost efficient to reduce file size of images on the server. That's the main reason for doing that imho.
But you could give it a shot and ask them directly:
growdiaries.com/contact
Maybe they will enlighten us all what format/resolution/type to use.
I just go with full resolution, maximum file size/quality and hope for the best. 😃😎💩