This procedure is for a PC with a mouse. Not sure it's the same for a mobile device, as I haven't tried that yet.
With your photos attached and as thumbnails at the bottom of your post (sounds like you have gotten to that point), have your cursor where you want to insert the image in your write-up and then click on the "Insert..." at the top left of the desired thumbnail at the bottom (I think it appears as you move your mouse over thumbnail). Where "Insert..." was, you will now have a choice to insert the thumbnail or full image into your text body where your cursor is placed.
Make sense?
