Once your photos are on a photo hosting site (such as Imgur.com; Postimage; Dropbox, etc.
Photobucket),
or on virtually any site, including this site's album such as eBay; Amazon Pinterest; Facebook; an online Pipe retailer; you-name-it, select the full sized image, then Control-click (Mac) or Right-click (Windows) on the image itself, then choose "copy image location" or similar words. Now paste that URL (the full web address, which should end in .jpg or .png) into the IMG box in the reply window of the thread you're posting to.
The site's album is also a good choice for displaying photos, and the same method works for obtaining the image's URL for copying into the IMG box.
There are other good illustrations and steps on how to post photos under "Latest Discussions" in different formats:
LINK to that thread