if you want to add photos from social media site to your post easily, take a look at the "</>" button all the way to the right of your icons. When you roll over it will say "Insert Embed."
When you click it once, you'll see over 10 icons: Facebook, Twitter, Google +, Linked In, Pinterest, Vine, LastFM, Spotify, Instagram, Indiegogo and Kickstarter.
Click again on the service you'd like to use and you'll see a field where you can cut & paste a URL from that service.
For example, if I want to share a photos from Instagram I will open up another tab on my browser, go my user page, click on the photo, copy the URL from the address field. Then return to post on the forum, click the </> button, click the Instagram button and paste the URL.
The preview option will show a placeholder until you create your post. Easy!
Here is Charley's explanation. mostly the same. go to your Instagram page, and click on the picture that you want to share, so that that picture/photo is on your screen, and not all your photos. On this forum click on the 'reply' button on the right - not the quick reply button. Write what you need to for your post. Click on the <> button and choose the IG button and click it.
On your IG page, with the photo you want to share on your screen, click the photo, and some letters etc will appear in the address bar, and they should turn blue (for copy). When I get to that bit, I click edit, then copy, go back to the forum with the IG photo ready to paste into the little address bar. Make sure your cursor is in the little bar, click paste.
Some 'stuff' will appear in the bar, and then into your post when you click embed. This will show up as your photo when you have pressed the post/reply button. This is what I do. It took me a while to do it in one go!!