The image.jpg here would be an image file in your chrome directory. object-position controls how the given image is positioned since it might have different dimensions than the original one.
But for the various other images - for example that in the sidebar - for them I don't see any obvious way to select them only in this community. For them you might need to just use the <img> src attribute which kinda sucks but should work. It would go like this then (this would also work for the main community page):
Yeah, I don't know if you can do better than that.
On another note, we may be moving this community to fedia instead - to here: https://fedia.io/m/FirefoxCSS or via this site. Not much there at the moment but thought I'd mention it.