There are two ways to attach images to your post.
If you have found an image online, you can copy the URL to the image and press this button:
and within the img-tags, paste the URL to the image. When you press "Preview", you should now be able to see the image in your post.
The other option is to upload an image you have saved on your device.
To do so, below the area where you write your post, go to the "Attachments" tab and press the "Add files" button.
Select the image you want to upload and press OK.
After the upload is complete, you can press the "Place inline" button and it will be inserted into your post.