TLDR: Copy and paste a picture's or Youtube video's full address (= hotlinking) to directly display (= embed) the picture or Youtube video into your post.
Attachments on this forum
This forum has an attachment functionality that allows you to upload pictures and other stuff.
Your space for attachments is very limited and you'll run out quickly. This forum is not meant to store lots of your data.
For serious posting of pictures and videos (and other big stuff), use an exter