The Blog Entry Topper feature has been around since sometime in 2011 or so. I’ve added it to all my themes since then. It is pretty simple stuff overall, and has been available to other developers to use as well. In fact you can find it on Github as well.
As for it appearing in an RSS feed – I’m doing nothing special in this regards. The image still resides in your blog post’s content. When the Blog Entry Topper does its thing it is only moving the image around on the page during page load to display it above the blog entry. This leaves it in place, right where you placed it via drag-and-drop, when you wrote your blog post as a part of the post itself.