Q: How can I transfer a single photo from my macbook iphoto library to my ipad photos?

You could use iTunes to sync the photo. You would first assign it to an Album by itself. Sync photos to your iPhone, iPad, and iPod touch in iTunes - Apple Support

Another method would be via a 3rd party app such as http://www.photosync-app.com

or via Photo Stream My Photo Stream FAQ - Apple Support

or you could email it to yourself

or use a 3rd party service such as Dropbox

Airdrop would be an other method but it would require a 2012 or later Mac with Yosemite installed.