images Image sizing between mobile and desktop

Site URL: https://www.divineautorestorations.co.uk/

Hi I am new to the forum, so be kind.  I am not a coder and take advantage of the templates and run 4 sites for friends, not on a commercial basis !

I know this subject has been covered, but I cannot work out how to adopt the answers to solve my issue.

I am heavily compromising the images I use on this site that is under development www.divineautorestorations.co.uk - the header images are hard to get to scale nicely on desktop and mobile - they look good on one but terrible on the other.  The template allows scaling in steps S / M / L etc - I suspect I need to add CSS ?

Is there a generic insert CSS I can use, or is it custom per image ?

Thanks

Anthony

Examples below

Hi. Which pages in screenshot?