WordPress automatically creates 3 resized versions of your uploaded pictures: thumbnail, medium and large. If you want to save some space and disable these there are two ways you can do it. The first is to put ‘0’ via Settings -> Media in your admin panel and these images will no longer be created.
However you can also do this programatically in WordPress which is especially helpful if you are running WordPress multisite or would like to release a theme or plugin. Just paste the code below in your WordPress functions.php file.