Problems with featured images and gallery previews

[RampantPanda]

Hi, I run a photography blog at senseofnothing.com. I have two problems, I guess they are separate.

1. Featured images. These are huge and don't even fit the screen sometimes. I tried to edit my theme's config.json as follows to reduce the image size used on tag pages:
   "featuredImages": { "sizes": "(max-width: 2560px) 100vw, 2560px", "dimensions": { "xs": { "width": 300, "height": "auto" }, "sm": { "width": 480, "height": "auto" }, "md": { "width": 749, "height": "auto" }, "xl": { "width": 1200, "height": "auto" }, "xxl": { "width": 1600, "height": "auto" } }
   , but then I run into errors when generating. I really don't know what I'm doing, but I guess I should also edit some other files.

2. The galleries. The previews shown on gallery page are really poor quality, maybe they use a smaller thumbnail are scaled too big. When I click on the images, they are ok in the lightbox. How can I adjust this?

Like I said, I really don't know what I'm doing editing these files. I tried to find the relevant info in the development documentation, but didn't find anything.