How to add background to an animation?

[binocthex11]

I have a GIF animation with transparent background. I want to put the animation
on a background picture, whose size is identical to the animation. If I add the background
to each frame of the GIF, the final file size will be too large.

So how to use ImageMagick to do that? Thanks!

[anthony]

Static Background - compose over a larger background
http://www.imagemagick.org/Usage/anim_mods/#background