There is another option:
Insert image via yaml in index.rmd, using the solution under Tip 3. Add a Logo in your title/header/footer at this blog post
The code looks like this - note there are two spaces after ) to put the title on a new line when rendered:
---
title: |

My title
However, when you hover over the image, it blows it up (to something it thinks is the original size? Can this "Hover" behaviour be disabled?
This also works with an image from the web (eg imgur link like https://i.imgur.com/7An2gTx.png) - just stick the link in the brackets.
The downside of a weblink is that is doesn't render in the Rstudio viewer.
"Note that the Viewer pane can only be used for local web content" Why oh why Ian Pylvainen?
OTOH, the weblink is preferred if you use the includes with html option, as the local link to my image got lost in the build process, while weblink was fine.
Note, I still think it is preferred that I could just have an image above a top level header without any fluffing around with includes or yaml headers. If you have a book, which is what bookdown is trying to replicate, you want to see the cover or something visual first, and that should appear before the top level header which is often Preface. I suspect if you go and look, you will struggle to convince yourself that seeing Preface above the cover image makes sense eg