Prepare two images - 512x512 and 32x32. Scale first one down to 32x32, subtract it from the second one and this will yield a arithmetic, per-pixel difference. Compress it. Now store 512x512 image and the compressed diff with an app. With these two, you will then be able to recreate original 32x32 (by scaling down 512x512 and applying the diff). And I am making an (educated) guess here that the compressed diff will take less space than the actual 32x32 image.