I'm a mediawiki developer. Its open source. You can certainly contribute things. For something like that the difficult part is convincing everyone that loading=lazy is the right thing to do for body images. There are pros and cons, and a patch changing something without a clear right answer isn't going to get merged without buy-in (i should note im a backend dev, so this particular example is not something i know a lot about)

Anyways, lots of mediawiki devs hang out on #mediawiki on irc.libera.org irc channel. If you want to get involved come say hi.