loading image -> detect when image is done loading

[edfialk]

Hello,

I have a web application up at http://niceguy.wustl.edu/openlayers
It's a comparison tool for different gridded emission data using
OpenLayers mapping.
Feel free to play around with it.

I'm looking for a way to have a loading message once the user clicks
submit.
I can easily make the message (or animation, w/e) appear somewhere
once the request is submitted, but my problem lies in making the
message disappear once the request is fulfilled.

In this case, the request is simply downloading an image.

Does anyone know if there's a way to detect if data is still
downloading, or if something has finished downloading, or any other
idea that could help me here?

Any ideas would be greatly appreciated.
Thanks!

-Ed