By default Windows 10 doesn’t show thumbnail pictures in Explorer. To show thumbnail pictures in Windows 10, open Explorer, click the view tab and click options. From here you can select the view tab (I think that’s what it’s called, its the middle tab of the options window) the tick the box to always show thumbnail pictures.
Why does Windows 10 not show thumbnail pictures by default? The reason for this is that showing thumbnail pictures makes the Explorer window run slower while its loading all the thumbnails. These days people store lots of their pictures on their computer and tend to keep them all in one directory. This will take a long time to load.