Play around with the "Special BBCode" icon, in the upper left hand corner of your "Posting Editor."
If you want full images inserted into your post, you will need to have the image hosted on a free hosting site. There are many. TinyPic.com or PhotoBucket, for example. Once you upload the image to one of those hosting sites, get the image URL and insert it into the "image icon" in your posting editor.
To post YouTube videos, just grab the video URL and insert it into "Media" of the "Special BBCode" icon, in the upper left hand corner.
Experiment with all of the features and have some fun. You're not going to break anything. Let us know if you have any questions.
IMPORTANT NOTE: The video shows how to post images from your personal hard drive/computer. If you want to post an image from the 'Net, just grab the image URL and insert it into the "image" icon. That's it!