Wanting to make wallpaper effect

hi,

Im trying to make a wallpaper effect in flash. Let me say its partially working, but not the way I want it to.

what I want to achieve:
- with 2 images on screen, i want my animation to narrow vertically- image1(frame1 to 20) and then expand-image2 (frm21-40) - like a wallpaper.

Heres a summary of what ive done:
- I have 2 images placed on stage: image1 and image2, both aligned and centered(hor/vert). Both images are 'broken apart'
- i have set up frame 1-40 for the animation.
- image1 has 2 shape hints a,b both on the upperleft and upperright of the image, respectively at frame1.
- image2 also has 2 shape hints, a,b however both on the lower-left and lower-right of the image edges respectively, motion tween etc.

The way it works now:
- the stage narrows vertically(with image1 in the background).....frm 1-20 and expands again(with frm 21-39, image1 in the background) and then image2 appears only at frame 40.

how do i make image2 to appear unfolding vertically from frame21-40 instead of image1. I want the screen to fold(frm1-20) vertically with image1 as background and it should unfold(frm21-40) with image2 as background.

thanks

use a mask over image 2 and animate the mask instead of the image.