A github profile README can be created by creating a repository of the form username/username. The README will display above pinned repositories on the user's profile.
However, some images don't display, and from some manual testing, it appears to have something to do with either dimensions or the image file size.
What specifications must an image meet in order to display correctly in an otherwise empty profile README? That is, a profile README that simply contains:
<img src="https://github.com/favicon.ico" height="240" width="480">
Note: here's how the README displays when it fails to load the image:
