Sync single photos from Mac to iPhone
Its difficult to give the best answer without knowing which photo manager you use and how often you'd to transfer a photo. For rare transfers, email and Dropbox are good solutions. 

For a more sophisticated approach, I use a smart album in Aperture together with a keyword called "Sync." Now whenever I add the Sync keyword to a photo on my Mac, it is automatically added to the Sync album, which is set (in iTunes) to sync with my iPad.