Hi there!
As you have noticed, Facebook carousels only support posts with two or more slides. You can find more information about Facebook Carousels here.
You may also publish posts using the image from the link by utilizing the link preview. In this case, Facebook wants to see the right properties in the metadata for this link. Do check this article as it explains what information Facebook link scrapper is looking for in order to generate a preview in the platform. This way the post "image" will be clickable on Facebook.
Alternatively, you may also simply schedule a regular image post by adding an image into the Publishing Window and copying your link into the text box. This way you will have an image post with the link above the image and the post image will be viewable as an image on Facebook, but users will still be able to click the link in the post text to see it.
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