Depending on your background image size, css background images can impose a high cost on the rendering of any page. This is variable but the more area you cover with a background the higher the cost - as the render engine for the browser needs to repaint larger parts of the screen. You can see this in action if you use Chrome and use the developer tools and the timeline tools.
A fix for your specific case of animating the images maybe to do a 3d CSS transform. IE don't do a normal transform but a 3d transform such as 'translate3d'. This will automatically trigger hardware rendering (ie your computers gpu will handle the transform) and offload some of the overhead from your cpu - and in theory make your animation much smoother.
This comes at a cost of course, especially if your dealing with mobile or other limited platforms. But its worth a shot.