To work with a photo or other item in the Photo Browser, you first need to select it. A selected photo thumbnail is highlighted with a dark blue outline.
To select a single item, click its thumbnail.
To select multiple adjacent items, hold down Shift and click the first and last item you want to select.
To select multiple nonadjacent items, hold down Ctrl and click the items you want.
To select or deselect all items in the Photo Browser, choose Edit > Select All or Edit > Deselect.