Rookie question: How does WP handle picture versions

In developing https://www.haroldthewonderdog.org for #2 son late last year, I made some mistakes initially on picture sizes (meaning that I didn’t pay attention to their size). I am currently running 4.2.3 and Divi theme 2.4.6.1 . After the initial set up, I now scale all pics before uploading. But I see, looking at File Manager in my cpanel, that even tho’ I rescaled/resized the large (1 to 2 to 3mb) pics down to 100-300k, it appears that WP keeps all versions. This results in an Upload folder about 499mb in size and takes my backup plugin 1 to 7 hours to back up, not to mention the plugin whining about encountering a large file size. My question is, I am about to go through the site and remove all versions of a picture from the affected galleries and re-upload a correctly-sized photo in their place. I’m hoping this will reduce the overall size of the site.
So, is that the way to accomplish this? Many thanks in advance. DD