The media icon is on GS.
When you go to reply to a thread, there should be a line of light gray icons on top of the post. For example, there is something that looks like an easel, something that looks like a chain, etc.
I don't know what half those icons are for, but I know media. It works mostly with Twitter, IG and YouTube. The media icon looks like two picture frames. To the left is a smiley face, to the right quotation marks. Click on the icon with the two picture frames. It will prompt you to insert a URL. Insert the URL. Then voila, a photo in the post.
It took me forever to figure it out, but now that I have I'm a picture posting fool when it comes to IG and Twitter.