It works this way:
[url=youtube link here]Text of link or link of Image here[/url]
Actually, you have to put the image link inside img tags inside url tags, otherwise, you will see the link as "text" not the image.
Example (thank you bones261):
[url=https://www.youtube.com/watch?v=vK2u4y7J58I]https://i.ytimg.com/vi/vK2u4y7J58I/hqdefault.jpg[/url]
Result:
https://i.ytimg.com/vi/vK2u4y7J58I/hqdefault.jpg