Image Border Help

Hello,
I’ve established a web site in WordPress which I’m very happy with. I’m now polishing some of it, and I’m struggling to find a method for putting borders around some of my images. This is typically for letters / emails / certificates which I’ve scanned in and want to publish, where it’s a predominantly white image (scanned A4 document) against a white background (my web page). I’ve been unable to find a current plugin to achieve this end, and would be happy to edit the HTML for any particular image (there isn’t a huge number for which I want to do this) to achieve my end. I’ve done some research and attempted a few times to edit the HTML as guided, but had numerous failures – I’m guessing that the articles were for earlier versions of WordPress.

Could anyone advise what I could do with the following html which I’ve retrieved for a typical image on my web site? I really don’t want anything fancy, I’d be happy with a simple 1 pixel width border in black. Is this something that’s possible, or do I need to learn about CSS classes and the like?

<figure class="wp-block-image size-full"><img src="https://www.ineededtobeneeded.com/wp-content/uploads/2022/01/MPS-Email-2.png" alt="" /></figure>

My web site is running WordPress 6.0 with Sinatra theme.

Any help would be greatly appreciated.

Kind regards, David