Animating a persp plot: non-constant scaling?

Hi all,

I'm making an animation of a rotating persp plot of some data.  It's easy,
really:  I just plot the persp in a loop over the theta parameter, saving
each plot, and then chain them together with ImageMagick (as suggested on
Paul Johnson's R-tips page).

But...  when you run the animation, because each graph is individually
re-scaled, the image is not the same size in each frame.  That is, when the
bounding box is facing you (theta=0), the plotted image is bigger than when
it's at 45 degrees to the front.

Here's a simple example:

thetas <- seq(0,360,10)
for (i in 1:36){
        png(paste("image", "_", i, ".png", sep = ""))
        persp(outer(seq(0.1,1,.01),seq(0.2,1,.01), beta), theta=thetas[i])
        dev.off()
}

then rename the files (1 -> 01) so that they're in order
(is there a clever way around doing this from within R?),

then animate with a command similar (on Linux) to:

(bash)$ convert -delay 10 -loop 5 image*.png animated.gif

Then open it up with something that likes animated gifs (I use a browser).
See how it wobbles?  Any clues?