fsuaterdogan
YouTube embeds work fine for my forum. I just pasted a video URL and it embeds normally. I don't think if this is handled by an extension, so you probably have to enable some extension from your admin area, however I did not find any extension for that so I guess it's part of the Flarum itself.
As of the images, the format is ![alt text](src)
.
The alt text
is text that appears when you hover on the image, so you can technically leave it blank. The src
is direct link to the image, such as https://example.com/image1.png
.
The images are part of the Flarum core, so you don't need any extensions for that. Therefore I think that there is a mistake on your side.
If you need more help, just reply and I'll be happy to answer 🙂