If the caching this isn't the problem, it must be with your page lifecycle. It's impossible to tell based on what you have so far, but the only thing I can think of is that your code in the question (which sets the image URL) is wrapped in a if (!IsPostback)
, so it would not be updated upon upload, but would be updated upon a full GET
load of the page.
Throw a breakpoint on that line and see when it actually gets hit.