Just found this description on another forum that uses the same forum software. It seems pretty simple to understand, and should supplement what Sheepherder has posted.
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"When you want to UpLoad a photo or file as attachment to a message, or when you reply to a message, scroll down to the bottom and click on "Manage Attachments", and a window will open up for you to UpLoading your files or photos. DO note that there are size restrictions for the files or pictures that you can UpLoading. It will tell you that in the window.
Now, if you want to show your picture in the message (otherwise it will show as thumbnails after UpLoading), you need to open up your picture. It should open up in a new window or tab, and then you want to copy the web location of that picture (
www.xxxx. etc.).
Now go back to the body of your message, and wherever you want the picture to be, type in: [img] and follow immediately with the location of the picture that you had just copied, then close with [/img], leaving no gaps within the expression. Hint: if your picture is already somewhere on the web, and is accessible (not private), you won't have to UpLoad the picture here. Just copy the picture's web location and you can place the image as described."
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I will only add that it is preferable to upload your photos here, because linking them from somewhere else often leaves the thread with no photos. Why, because many of the professional people who belong to this forum access from behind corporate or government firewalls that filter out any internet photo storage websites, so PLEASE load your photos directly to this website. The other issue is that external storage websites often reorganize their directory structure and any links that you may post to external photos may go dead, and once again your message will appear without the photos that you wanted to associate with it.