I found this out on accident, but this is worthwhile for everyone I think.
Step 1: Upload a picture via the site like usual.
Step 2: In the attachments popup, click on the uploaded picture. It will open in a separate window.
Copy the address of this picture.
Step 3: Click on the insert image icon and paste the picture link.
Use the preview post button to see your uploaded picture in the post.
Step 4: Well done! Enjoy!