The latest edition of TweakUI let's you do it without needing to go near the registry.
Open TweakUI, click on 'Internet Explorer' to expand the tree. Then click on both/either 'Small Animation' and/or 'Large Animation' and then select 'Custom' and browse for your pre-made bitmap replacement. Make sure you make your bitmap according to the 'Description' provided in the TweakUI window.