I remember a template for technical design documents that had a placeholder to insert a header image. Any image, could be totally unrelated to the document content. The template explains that it's a sort of mnemonic to identify the document, since everyone is expected to use the same template, and all walls of text roughly look similar to each other from a distance.

So with some header image, reader might be able to recall "it's that Clice post with Yami on top" or "it's that other Clice post with Kessoku Band".

That doesn't explain why anime aficionados (and furries) have such images in their posts vastly more often than for example people into marijuana or craft beers or motorcycle racing.