Make the post.
Click to edit the post, then insert the picture URL into the box.

For some odd reason, if you click the "add picture" box under the text box on any Apple device, it only wants to post one from your computer/phone/iPad. But you get different options when editing. There are boxes to paste the URL directly into.

My Windows machine doesn't have any problem pasting a URL into the original post. It's just Apple devices that want to insert from an image in the device's photo gallery.