You cannot load photos directly from your computer to this form. The photos have to be linked to an internet photo hosting site. Once uploaded, the photo is given a URL. Using the "Advanced" posting the html "tags" are created before and after the URL for your uploaded photo. If I tried to put the tag in here, this system would think it's a link to the photo, but leaving spaces, it should work to display. In the real link being posted, it would look like this in the edit window (without the spaces): [ i m g ] photo.com/123.jpg [ / i m g ].
Photobucket.com is on such photo hosting site that 100% free. Create a user ID, log in, and simply click and drag your photo to the "library" or directory you create. Once it's uploaded, click on the image to bring it up and on the left is the direct link. Copy this URL and paste it in the RV.net "Edit" window (utility for photos) or type the before and after tags yourself.
It's really pretty simple, and pretty fast once you done it a time or two. Photobucket also give you the ability to edit your photos some, which is nice to resize before posting on RV.net.
Hope this helps. Good luck. But you cannot upload from your computer directly to this site. You have to use a photo hosting site on the internet.