Hi @neodjandre
Please keep in mind that the second method needs the fl_afeat field to be a URL field, not an image field.
If you really need the thumbnail to be an individual image object, you need to upload and process it first. Unfortunately, this is out of ACF topic. Kindly check this page to learn more about it: http://wordpress.stackexchange.com/questions/141172/how-can-i-pass-an-image-file-to-wp-handle-upload. If you need further support regarding the upload issue, kindly visit WordPress community instead.
I hope this makes sense 🙂
