I think that the best option here would be to disable the generation of additional image sizes on adding an image to the media library. Doing so is fairly straight-forward:
Visit the Settings > Media page of your WordPress dashboard. Under the Image Sizes section, change all of the values to 0.
Save the changes. This will stop WordPress generating thumbnails, medium, and large sizes of every image you upload.
You will also notice that when you go to insert an image, the “Size” dropdown box is missing.