Images and videos are not actually on this forum. They are hosted somewhere else on the internet. Basically it is like a hologram on Star Wars. The image is being projected from somewhere else. If you have a document that you want put in a post you must first upload it to the internet. You may use photobucket, flicker, googleimages, etc. You can do a search for "file hosting" or "image hosting" and take your pick.
Once you have your document on the internet then you need to copy the html code. In photobucket when you move your cursor over the image it comes up automatically with four choices. One of the choices being the html code. I don't know how to find it for other image hosting sites. So if you use one of them you need to find the html code or bbc code and copy and paste it to your post. When you hit preview it should show up.
Videos can be done the same way by using the embed option. Just copy the embed code and past it to your post. Do keep in mind the forum rules about appropriateness. Sometimes youtube can have related videos show up after a video has played. Make sure there is nothing inappropriate in the related videos. If there is and you still want to post the video let us know and we can help you. There is a way to suppress related videos.
If you have any more questions please reply to this post.