Imagine you have a book filled with pictures. A image viewer is like a special kind of book that helps you look at all the pictures you have on your computer.
You can use it to open up pictures and see them on your screen, just like you would with a regular photo album. Image viewers also let you do things like zoom in really close to look at details, or rotate the picture if it's not the right way up.
Some image viewers even have special tools that help you edit pictures. This means you can change things about the picture, like making it brighter or darker or drawing something on it.
Overall, an image viewer is a helpful tool for anyone who likes to work with pictures on their computer. It's like a magic book that lets you explore all the pictures you have in one place.