If you are using a large image in websites, automatically loading of the web page will increase it will leads visitors to skip from your web page and navigate to some other websites.
So its better to show thumbnail image which link to larger sized one. So when loading the web page only thumbnail image will display which is loading quickly. For this reason most of the people maintaining two images one for thumbnail and larger sized image.
But here also we have to maintain two separate files and linked between each others.