To add a photo to the forum gallery you first access the gallery by clicking on the gallery link next to the camera icon at the top of the forum.
Next you click on the 'add file' link in the gallery.
Add a name for your photo and description(optional) and then you need to choose what category your photo belongs in from the drop down box that is circled. If it's a photo of yourself it would go in the 'member photos' category.
Next you need to use an imagehost to host your photo. I recommend tinypic.com or imgur.com. You need to upload your photo there and use the 'direct link' code. The code should always end in .jpg or .png in order for it to work. Just copy the 'direct link' code.
Paste the 'direct link' code where it says 'Enter a URL to media'