I found a formula for making 2 images proportional on a forum the other day.
The formula is: H₁ / W₁ = H₂ / W₂

Example:
if (image1.height > image1.width) {
newWidth = (image1.width / image1.height) * preferredheight;
image1.height = preferredheight;
image1.width = newWidth;
}else {
newHeight = ((image1.height / image1.width) * preferredWidth);
image1.width = preferredWidth;
image2.height = newHeight;
}