the answer in the MDN docs.
http://oli.jp/2010/css-animatable-properties/#background-image
background-image
This is still a little up in the air, with “only gradients” in the
current Working Draft, no background-image at all in the current
Editor’s Draft, and “Animatable: no” for background-image in CSS
Backgrounds and Borders Module Level 3 Editor’s Draft. However,
support has appeared in Chrome 19 Canary, and this is something that
designers want. Until widespread support arrives this can be faked via
image sprites and background-position or opacity.'
At this time I have found that nobody actually supports gradients (Chrome 17, FF 12, IE9). Only Chrome supports background-image (that's why it doesn't work in FF for me).
To Animate Background
Try this