Interesting I have the opposite view. From intuition I would think there is an unbounded set of simple compression algorithms. You can probably make much more effective simple compression algorithms by making basic assumptions about the type of image.
This is patently so. Taken to absurd proportions, you can compress a specific rendition of the Mona Lisa with just one bit. 1 means Mona, and 0 is undefined behaviour.
Are we about to talk about Kolmolgorov complexity? you're damn right that we're about to talk about Kolmolgorov complexity! :)
Taking this a step further: 0 means that all subsequent bits contain an arbitrary PNG. Now your algorithm encodes any image, and still does Mona Lisa in one bit.
It's also almost as effective as PNG (plus one bit) for non-Mona Lisa images.
Well, this is a combination of simple approaches which have all been used before, though put together in a nice way. And it's not benchmarked against modern formats like lossless webp or jpeg-xl.