You can use that little yellow button in the space you type the text in and paste the image link in there. Or you can paste the image link in the text space and put [img] [/img] around it.
For a YouTube video you put [yt] [/
yt] and put inbetween them, the link of the video from the "=" part.
Also you don't have to put that "yt" in bold, it just wouldn't work normally
I'm not really sure if that makes sense but yeah.