Dyno - Sounds like you have it covered. It would be relatively simple to do, and I think your idea of making the final image visible is a good one.
Then in the onLoad event, you'd want to check the last page, and if you want to show the effect you call shrinkImg();
bigImg = findElementById('bigImg');
smallImg = findElementById('smallImg');
smallImg.style.visibility = 'hidden';
finalWidth = 50;
finalHeight = 50;
numSteps = 10;
stepW = (bigImg.style.width - finalWidth) / numSteps;
stepH = (bigImg.style.height - finalHeight) / numSteps;
while (bigImg.style.height > finalHeight)
bigImg.style.height -= stepH;
bigImg.style.width -= stepW;
bigImg.style.visibility = 'hidden';
smallImg.style.visibility = 'visible';
Also, you need to set the id tags of the images on the page to bigImg and smallImg, accordingly.
Lemme know if you have questions.