Background images and

Firstly greetings. Seems to have potential.
Was looking at grav. A question or two that may have been asked. Though I didn’t see the answer from searching posts:
I’m considering Grav to use for a Blogish thing- as I’m traveling and like that it’s small light weight and, why not?
-I didn’t see a way that stood out to make a gallery for pictures- For instance withwordpress it has a bunch of ways to manage pictures- and I can tweak it’s PHP to taste, and I can tweak a theme to taste.
Is there a way with the antimatter theme(for example) to put in a background image? I may have missed something obvious.

Grav has super powerful media functions that let you cache and manipulate images. That combined with a custom twig template will let you easily create any gallery or slideshow you could wish for (way more powerful than wordpress here!)

Also there here is an example recipe for doing a gallery

And another recipe for a simple slideshow

These should give you a little headstart to realizing your goals. Let us know if you need more info!