1. You'll need a Gold Membership or higher to sell on the forums.
2. Once you have that, all sales threads go in the Exchange.
3. To post pictures as a registered user you need to upload them to a photo hosting site. There are plenty of free ones that are adequate. Photobucket, Imgur, etc.
I use Photobucket and it's very simple. Upload to the site and then copy the IMG link provided and paste it into your post.
If you have a paid subscription (Basic Membership or higher) to the site, you can upload photos as attachments directly from your computer.