It is great that you do the prefetch for URLs and can display that. Those running other forums are challenged by this issue. Unfortunate that you must save first. Javascript could take care of this nicely as soon as you hit enter, as expected.
You have helpfully provided a default size for videos. But why can't we control the URL embed (or other) image size, either globally or individually?
We are looking to create a visually consistent and appealing environment. The image pre-fetch from a URL can be huge, making it not useful within the post.
Another case where there is an established model for how to do this but you folks made design decisions that bear no relationship to the reality of how someone would use this - make take a look at a FB page one day...