When you hit the reply button on the forum while using a mobile device, you should see a little blue box with a little landscape image inside it on the bottom right corner of your screen. Tap this and choose the “Files” button or whatever app destination that your pictures are stored in. Then just select the right picture, and it should load a string of text (your picture) into your forum post.
If you’re using a PC to access the forum and have your AR screenshots downloaded onto your computer (like what I normally do), you can insert images to the forum by pressing the “Upload” button—it’s the same mountain landscape logo that you’d seen on your mobile device. Then follow the prompts to locate your photo of choice and upload it to the forum.