Photos Shared Album - Can you tell who posted photo ?

You can- there's two possible ways. To see a certain photo's poster, simply open Photos, then navigate to the shared album. From there, double-click on the picture you want to check, then click the speech bubble icon on the bottom left. It'll bring up the name of the poster.

If you want to see an overview of posters, click the dropdown arrow under 'Shared Albums' in the sidebar, then choose 'Activity', as seen in the picture below-

This will show an overview of all the people who posted photos.

Hi.

that is not possible unfortunately