There is literally a button in the post editor to add images. You were trying to link the whole album, which is not possible unless there is a specific addon for that. So you don't use html, but the board syntax "[.img][./img]" (without dots) to directly link the images.
[.img]https://i.imgur.com/kPdU0mp.jpeg[./img] becomes
alternatively, you can link to the album URL as a simple weblink
https://imgur.com/a/cIMIz4N