Leaking details about the user in web application

Hope I will be able to describe my question, because I don’t know much about IT security.

Imagine a web application, which is used for getting og tags from url. User can submit a form, which has single url input. Server opens the url in a headless browser, get og:title, og:description and og:image values and sends it to client.

I was told that if og:image is not cached on server, linking the image like that will leak details about the user who sees the preview.

I don’t understand what details can be leaked. Can anybody explain this issue a bit?