Consider a social network in which the users upload their images (cover, profile etc). Each image need to have different sizes (ex: large, small, supersmall etc).
The upload is done by a API REST json request which contains the file content and some metadata.
The json response is the different urls for each size and the web app immediately uses these.
I want to have fast response (not to process the image and create all the sizes then to return the urls in the API response).
Problem: The web app shows immediately the photos after the api response by using the urls returned, so right now I need to return valid urls that contain/return images.
PS: I can make changes both in the backend and the client web application, so your suggested solutions should not be limited only to backend changes.