It's simple to use images for tracking purposes (which is why many email clients hide images by default)
More use (benign) cases with sharing images on nostr I can think of
- analytics (how many times has my post been seen? What countries do my users live in? What clients do they use?)
- A/B testing: display 2 different images and see which has a better engagement rate
- put image behind a paywall (zap post to see image)
- localisation ( for text inside image, determine most likely language user is using)
- experiments (e.g. display different images based on readable metrics, e.g. display ✨ for Amethyst users and 💩 for all others)
- countdown (use a gif and the current date to display a countdown animation that always stays up to date)
- display real time data