Sometimes the easiest way is so near. With the tool iDevice Manager Pro-Edition can modern HEIF, JPG fotos or PNG image files be sent without iTunes to the iPhone and iPad. Either per drag 'n drop or via the adding button in the iDevice Manager. Which images are synchronized is doesn't matter. Par example: holiday photos, photos of friends, of the last festival, the Holi festival, the last party with friends, of your dog or your cat. With iTunes you can just synchronize complete directories and that is often not needed. The iManager receives a message from the iDevice Manager, when new pictures have been sent and the iManager App picks up the new data. Who thinks spicy photos could get in wrong Apple mobile phone, can be soothes. Because the iDevice Manager Pro-Edition encodes the pictures with a password, which first the iManager App told the iDevice Manager. After the NSA affair are such high security features necessary. Whole process can be seen in a graph at the iManager description page.
New photo received
History of received items
New photo in the iPhone photoalbum