Its the image of a bunch of book covers. Better to have them together in one image than separate images.

https://joinbookwyrm.com/static/images/book-grid.png

It features several photographs, it probably should've been a jpg at least.

Or a webp, much preferable for the size savings[0] if its not an image the end-user would ever want to save.

[0] https://developers.google.com/speed/webp/

Wrote them an email