I worked at Amazon for a while as an intern, but I didn't see how a ton of the external facing stuff worked like this. So in some ways, this is mostly speculation.

I notice one big thing about image-based announcements: I'd rarely hear about the publicly first, and I'd definitely not hear about them internally in an official way. Leak-proofing things.

I suspect the reason for images is because it's simply harder for PR/management types to make a decent looking webpage that works in multiple browsers. It's way faster to export an image from whatever software they use to generate it.

And I suspect it avoids deployment headaches.