I imagine that because they have such a large volume of images and different combinations (20% off in one country, 30% off in another, ..) it makes sense for th

I imagine that because they have such a large volume of images and different combinations (20% off in one country, 30% off in another, ..) it makes sense for them to create images only when requested. They probably cache the generated images though. In addition, I think that any given day a small percentage of Amazon's products are viewed so it may not make sense to generate an image for every possible combination. They may also have a longer term view and plan on using this graphic manipulation in another product.