Thanks! That pointed me in the right direction.
I ended up using the following:
Redirect 307 /images/banners/imu728x90.jpg http://www.mywebsite.com/images/banners/imu728x90.gif
Since there was only 4 files I needed to do this to, this seemed to be the best way.
From what I could tell, the 307 Temporary Redirect status seemed to be the best to fit this, as it is temporary, and will change back in the future.