2500px is a very large image and sometimes flashing can occur if browsers can not render the images fast enough. However, I am still a little surprised it is happening. What machine/speed/OS are you using? Can you send a zip of the .hype document?
You could try disabling 'Use WebKit graphics acceleration' in the Document Inspector, but this would not affect Firefox. Another performance trick would be to check 'Position with CSS left/top"
There are some cases where server directives can tell a browser to not properly use a cached version of the image, but this is a more rare cause of flashing and wouldn't happen via the preview.