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, 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.