If you upload an image with WordPress, it creates additionally two thumbnails and stores them in your upload folder.
Many themes and users make no use of these thumbnails and there is a way to disable this function.
Go to Settings -> Media and change the values for Thumbnail size and Medium size to zero (0). Just enter 0 in the textboxes
Thumbnail size Width 0 Height 0
Medium size Max Width 0 Max Height 0
Leave Large size as it is and save changes.
On a new upload via WordPress there will be NO thumbnails for this new file in your uploads folder anymore.