Shamelessly stolen from Steve Otte at the Model Railroader forum.
Whenever someone new joins the Forums, they often have trouble figuring out how to post a photo. Here's how.
Step 1: Prepare your photo. It should be saved in JPG or GIF format, and no more than 1 megapixel in size (approx. 1000x1000 pixels).
Step 2: Upload your photo elsewhere. You can't actually post a photo in a Forum message. What you can do is post a link in your message that calls a photo posted elsewhere. So you'll have to put your photo somewhere on the Web that's publicly viewable. Many Forum members use Photobucket, Imgur, Flickr, or their personal websites. Whatever hosting site you use, make sure your settings allow your photo to be viewed by anyone, or all you'll get is a broken-image icon. The Model Railroader User Gallery won't work, because it requires you to be logged in. Any site that requires a login won't work, even if visibility is set to Public. That includes Facebook, Twitter, Instagram, and the like.
Step 3: Copy the address. Right-click (or on a Mac, control-click) to view the image by itself, not embedded in a web page. Copy the address of the photo from the address bar at the top. If it doesn't end in ".jpg" or ".gif", you're still looking at the picture on a web page, not the picture itself.
Step 4: Start your post. Create a new post on the Forum as usual, using either the Reply function on an existing thread or the blue "Create a new discussion topic" button in the Forum list view.
Step 5: Insert your picture. Click the "Insert/edit image" button above the message window. It looks like a picture of a mountain and the sun. Click it and paste the image URL you copied into the "Source" box in the window that comes up. Then click OK. Repeat and add text as necessary. Then post as usual.