My guess is that these are events others have boosted that live on relays you are not linked to. You are on relay A,B,C your following someone on relay B,C,D,E,F who boosts a message from someone on relay E,F,G,H...

they're uploaded somewhere, were you able to open the links and see the images in a regular browser? if so it might be an issue with amethyst, otherwise it's an issue with the image host used.

images are uploaded to separate hosts and are kind of malleable, therefore not guaranteed to stay around forever.