V3 was the wild, wild west. It was solely on Facebook to go to your website and grab a random image. The image may not even have been related to your topic, it could have been someone's profile photo, an ad, etc... It was pure luck that you were able to get it to be something related to your topic 😉 .
V4, we changed this so we are now suggesting a photo to Facebook (and others) via OpenGraph to grab this photo. Typically, it is the first attached image in the topic, otherwise, it will fallback to the default sharer image.
There may be third party plugins out there which may allow you to have better control if this is something you desire. However, this is not something which we support.