This is the most critical thing ever.
When loading up any project page, the menu in the sidebar appears at the top then pops down when the project image loads. I don’t mind you moving the cheese! I love the new cheese! but cheese that moves by itself? that’s worse than this extended metaphor!
If you know the height of the image you could probably use a margin to keep things in the same place as the page loads. I guess that you already know the default image height so you could probably just use that to cover most cases.