Hi @uwelang !
Thanks for trying out the app. Just some little informations (we still working on putting together a real tutorial / FAQ to explain all the features).

1. For YouTube videos to be embedded, you need to put an https link (steemit doesn't convert http links to iframes), so when you post on Twitter, you must make sure to have put the https URL and not the http one. here it's an account link, so there won't be any embedding, but it's just to inform you if you're planning to put some videos in your tweets.
2. To add a custom bannier (or anything in fact) in the beginning (or the end) of your posts made via share2steem, you can go on the website -> Social networks -> the button "manage" on the account -> and there you can customize the body of the post (explainations on how to do that are on the page).

Welcome aboard !