iTunes question...what am I missing here guys?

[TurboTravis]

Trying to use iTunes to move pictures and videos from my PC to my iPhone 7 Plus. You go to 'sync photos' and select the folders you want to sync. Ok, easy enough. But once you do it again, it erases what you did the first time?! It's like you get 'one shot' at it. What am I missing?

Thanks,
Travis

 

[SwitchBeach]

Are you syncing the same folders on the computer each time with the pictures from before STILL in the folders?