Some photos are already selected.
To select a photo, simply click on it; an orange frame will appear around the selected photo. The "Cover photo" and "Change caption" buttons appear on all selected photos.
To deselect a photo, just press on it (only if it is selected, it must have the orange frame), the buttons will also disappear.
Can I see how many photos I have? A counter of the number of photos selected will appear at the bottom right of your screen.(in blue)
When you click Validate. You can go back to add or delete photos.