How do you highlight multiple photos on mac?

Answer 1

If by highlight you mean select:

To select a block of items click on the first one and then hold down the Shift key and select the last item. All items between the two click will be selected. Alternatively click on the first item and then drag the pointer over the other items.

To select multiple items not located together in a block hold down the Command key and click on all the required items.

This is standard Mac selection procedures and will work with any kind of file.