I use Photoshop CS5 to create animations. You create the different frames as layers, then set it to "tween" (auto-create the missing frames between layers) to create the animation from point A to B. I usually start with more than 2 frames though, especially if it's a somewhat lengthy or complicated animation I want to show. For this purpose, you could start with 3 (tilted left, straight, tilted right) then see if tweening A > B and B > C creates the depth you want. You can add as many frames as you like and specify the delay between them to make the animation as smooth as possible, and it helps to have the layers be exactly straight so you don't get a jumpy animation.
If you don't want to go with Photoshop, you could try using the GIMP (GIMP is free, and does many things Photoshop does). I've never used it for animation but there's a tutorial here:
http://www.gimp.org/tutorials/Simple_Animations/ - there are also a lot of online GIF makers you could try out to see if they'd work, although I think some may have size or frame limitations. If you do go with Photoshop and would like help, just holler :)