In order to post a photo in a forum, first you need to place it on a device which is connected to the internet 24 hours/365 days. Those devices are called servers (imgur, photobucket, pipesmagazine album, etc.). After placing your picture in a public server you will need to obtain a path to the picture so you can provide others with this path (so they can go and see the picture). This path is called a link. Once you have the link, you come to the forum, click on image button, paste the link and click ok. By doing this, you simply tell that you are going to post (embed) image and forum sees it that way. If everything is correct then your photo will be exhibited. What you are trying to do is directly posting a photo from your device and forum won't support it. To share a photo with someone you either have to send it to him (via email, message, messenger, etc.) or you have to place it on a server (free public servers, facebook, instagram, etc.) and provide the path to it.
