To expand your stack and see all the photos in it, just click directly on that little number 12 in the upper-left corner of the thumbnail (the expanded view is shown in Figure 4), press S on your keyboard, or click one of the two thin vertical bars that appear on either side of the thumbnail. To collapse the stack again, just repeat the action.
To add a photo to an existing stack, you can use either of two methods:
- Click the stack first. Then Command-click (PC: Ctrl-click) the photo(s) you want to add to the stack (to select the photo or photos). Then press Command-G (PC: Ctrl-G) to group the selection with your existing stack.
- My preferred method is to just drag the photo I want right onto the existing collapsed stack and drop it there to add it to the group.