Then on Attachments + Add Files - Photo library. Your own photo stream will appear.
Choose the pic you want from your own photo stream. You’ll probably need to reduce the file size to around 450 by 350, or 350 by 450 for landscape. Either use the software on your device, or download a free image reduction app. If on a phone, choose Medium for size.
Place the cursor within your post, where you want the pic to be. Click on Place inline, below the text box.
Then click on Preview to check what has happened. The preview box will appear above the text box and will show the picture, not just a text file link.
If happy with it, click on Submit under the text box.