As per the quoted post, while i recently gained access to attachments, the general userbase will very likely not do so and best to use some image hosting service and then embed them using the Insert Image button in the toolbar i.e upload it to e.g. Imgur then grab the URL with the image extension (like .jpg) and then put it in the box that the image button opens, alternatively there is also the Media button which can also work on imgur albums if you happen to have several images and don't want to insert them all individually
Uploading images straight from iPhone storage is not possible, but you can attach internet-sourced image links or upload your image to web services like imgur or imgbb to get image link