When you connect your phone to a public wifi, is it possible for someone to see your photos being uploaded to iCloud?
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.