imgix is an image CDN service (not free) that offers this feature and more. Not usually used for personal projects I guess since there is no free tier for it.
Imgix is great, but expensive as a cdn + image size on the fly company. I have been testing then for ~6 months for thumbnail generation but will be transitioning off soon. The cost is running about half of my S3 storage and transfer bill for only generating thumbs.
It's hard to justify not just creating my own thumbnails and stashing them on S3 for my use case.
It's hard to justify not just creating my own thumbnails and stashing them on S3 for my use case.