Can't group two pictures

When you try to group two pictures and the group option stays grey then one of the pictures will be in a placeholder.

If you insert a picture in the layout Title and object, PPT automatically inserts it in the placeholder even if you did not click the picture icon in the box. This is by design and can't be changed. So if you want to group two pictures make sure you use a slide layout like Title only and insert your pictures separately.