When you connect your phone to a public wifi, is it possible for someone to see your photos being uploaded to iCloud? When your phone backs up the photos, can someone see the photos?
Short answer, no.

Essentially, not only is the transmission encrypted -  see
iCloud security and privacy overview
but also since iOS 8 your phone changes its MAC address pseudo-randomly, so it can't even be identified as 'your' phone for a lot of the time.

Every time a new potential exploit is discovered, there will be an update to close the door on it - see About the security content of iOS 8 for some eye-glazing detail… ;)

Some of this is not actually published by Apple, as the last thing they want to do is give away how they are trying to ensure your security.