Generally what blog post authors do is that they upload large image files taken from their high resolution cameras everyone and after uploading WordPress crops them in different sizes, a few are used in your posts but those large files are hardly ever used and they take up a lot of storage space on your server. You can easily save your storage space by using a simple WordPress plugin called Lean Media.
Lean Media allows you to automatically delete very large image files after you generate the thumbnails. Largest thumbnail gets saved in place of the original image just in case you decide to generate thumbnails again and if your users are kind enough to upload small images then the plugin leaves such images intact. Simply install and activate the plugin then visit your admin area Dashboard -> Settings -> Media page and check the option for deleting large image files.