I’ve given this a try and the 300px x 200px ruins the compact layout of the sidebar widget. It also doesn’t fix the problem of the broken images. They are still broken.
It also messes with the overall layout of the page and the images, not just the sidebar. If the size is set to 50 x 50px (the original setting in WP Show Posts) then the images are still broken (obviously), the home page images go tiny, and the page just looks a mess.
I added the CSS in the customiser CSS area. I assume that was the right place?
Yes images are displayed when the height and width are omitted, but they are large images and I don’t get the title at all, and it’s definitely not the layout I’m looking for – but images do get displayed.
How can I get it to display the way it is supposed to?