SEO for images in image gallery

Hello.

I'm a photographer and using Squarespace for my website.
Therefore I also have different galleries there.

Now I thought about it and came the to conclusion, that my images should be displayed by google.

Right now my images just have the standard name after editing, there is no description or something.

What's the best to get them findable via google?

I think usually "alt-text" would be right, but I can't find the option.

Does the image name help with getting indexed? What about the "description" you can set for every photo in the gallery?

Is it possible to rename the photos after uploading?

Thanks!