Wikimedia Bugzilla is closed!

1: Initially load the image that you clicked (together with the spinner?)
2: Use new Image (or createElement img) to start loading the url of your high res
3: Resize the thumb to your intended dimensions, or alternatively, draw a gray line frame with a cross on the diagonals with that size
4: Switch out the two image objects

This should be easy to do and substantially reduce the perception of slow loading I think (At the moment, i fear this problem would draw a lot of unnecessary beta feedback and create concern in the communities)