Can someone (owner of public nw or intruder) perform man in the middle attack and steal the photos while they are getting synced over public wifi? Assuming they don't have any control on my cell phone, just by sniffing the packers, can someone steal the photos?
All services at google are using secure transport layer (TLS).
That means all the data you send or receive from google's services are encrypted with strong cypher suites.
If some one capture your data, he will not be able to decrypt it.
Please see google documentation here: Google Infrastructure Security Design Overview