Currently, closing expanded thumbnails can be a bit of a weird experience. To expand thumbnails, you simply tap the original thumbnail which is nice and easy. However, this creates a natural expectancy that tapping the thumbnails again will close them all again. In addition, having a small button at the top of the list in the center is harder to find and reach compared to just tapping one of the thumbnails.
Arguably, allowing users to tap any of the expanded or the original thumbnails creates a simpler and more natural user experience.