If you want to post a Picture or Video depends on the way you want to put it on the Forums.
Underneath the box you put your comment in there will be an Attachment Feature at the bottom of the comment box and will look like a paper clip. Now attaching files through that method will post to the Forum but only Premium Members will be able to see it and Regular Members that aren't Premium can't see it.
The other Method is through Third Party Upload Servers. For pictures http://picpie.org/ is a very good upload server. Just upload the picture, put the checkmark in the box Not Suitable for Family Viewing then use the embedded codes and choose BBCode Full copy that link to the comment box on the Forum and then Post and your Picture will appear so every user can see it.
The same goes for Videos but you have to use an Upload Server such as https://www.myairbridge.com/en/ or http://www.zippyshare.com/