Can I use a custom responsive header?

Hi,

A couple of questions about using srcset and responsive images in the header in the Smartline Lite theme.

First, a bit of background…
My new site is https://bell-tower.org.uk (have just moved over from an old hand-coded HTML site)

Using CSS I have deliberately limited the width to 960 pixels because I am reusing the graphics from the old site, which had that width. I expect to increase this when I have time to produce some higher resolution graphics.

Now my questions:
1. It appears from the readme and a superficial look at the theme code that Smartline Lite supports responsive header image widths using srcset. Does the theme automatically generate these (if so, I haven’t seen any alternative images in the library)? Would the fact that I’m limiting the width of the site affect this perhaps?
2. My old site (https://rgneighbours.elementfx.com/bell-tower) used a different header image on small screens and I would like to do this again if possible. My original idea was to manually upload alternatives to the smaller header images to replace auto-generated images – would this work? Or is the best alternative to do it using CSS?
3. When I run the site URL through the W3C validator it shows the following error:
When the srcset attribute has any image candidate string with a width descriptor, the sizes attribute must also be present. [From line 287, column 7; to line 287, column 358]. Would this error be related?

TIA!

David.