Windows temporarily stores the icon info in RAM while the computer is running. And after awhile the info gets stale, and the computer choses the wrong icon for a file.
You can right click on the desktop and chose Refresh, and it will usually snap all back to normal.
Sometimes you will have, for example, the icon for Adobe Acrobat substituting for all your jpg icons. And even refreshing will not help. Simply reboot your computer, and all will go back to normal.
If you are using your computer alot, all day, it is not unusual to have to restart several times, and it is a good idea.
If you leave your computer on all day and night, never shutting down, first thing in the morning, reboot. Do a complete shutdown, and wait a minute, and boot up.