No problems. What made me think of it wasn't so much trying to avoid paying for the service, but because I'd want to run a cache for speed of the initial page load. If you are serving your images from a CDN that would be compatible with my goal to keep the site fast.

I don't think anyone would "could comb through the content and replace the image URLs" to save a few dollars, as the development effort would far outweigh the cost savings. So maybe you are OK there.