Better image resizing capabilities

[MoshiKoi]

meta:289632

There's currently no easy way to resize large images to make them fit better in posts, and so extremely large images can take up almost all the screen real estate without a way to fix it other than locally resizing the image or manually writing HTML.

We might want to make this more streamlined for users. There are some markdown extensions that support image resizing, such as https://www.npmjs.com/package/@mdit/plugin-img-size, but that doesn't render on vanilla markdown, which is possibly an issue.

[cellio]

Kind of related (when we fix this issue we should update this meta post): https://meta.codidact.com/posts/290154 .