You are interacting with lemmy through some HTML rendering engine. Either you are using a standalone browser or you are using an app that uses the OSes HTML Engine. That is how it works.

Of course, lemmy and piefed and all the apps using the API to fetch content could just add a white background to the image-container. But then, if the image does not load for example?

Also, no, this is not UI design, this is the image creators job. If you provide an image, provide it in a way they can be used without assuming page backgrounds. MediaWiki itself has some lengthy recommendations to be darkmode save.

The background color behind transparent images should never be connected with the user interface color scheme, the background behind transparent images should always be white.

No, it should not, remember dark-mode memes? How about, i have we website, and i want my users to be able to switch between modes, should i avoid transparent images, if i do NOT need a white background (for example: Logos, clipart, cutouts .....)? If i know what image it is, and THAT it is transparent ok. But how does the fucking CSS know that the image is transparent? It doesn't!

As those projects are opensource, feel free to contribute.