After some investigation they're several things you can do depending on your requirements. A demo application is available here.
Part 2 is here: http://apex-smb.blogspot.com/2009/04/apex-interactive-reports-customize-wait_28.html
Before we go over the solution it's important to review where the image is located and how it gets toggled on and off. The image is stored in a span region which is hidden by default. The code looks like this:
If you want to manually make it visible you can run the following code:
First Let's build a IR page with the emp table:
connect by level <= 3
The easiest option is if all you need to do is change the image:
The new image, processing3.gif, is part of APEX as well. You can reference your own images if you'd like.
In Part 2 I'll go over changing the actual function that shows and hides the loading image. This will give you complete control of the loading message.