WordPress creates multiple versions of every image you upload by default. This adds extra files to your uploads folder, which is pointless if you’re not using them.
By default WordPress creates 3 different sizes of your image when you upload it to your site:
Looking at your uploads folder through FTP you will see the different versions created:
If you want to stop WordPress from creating these versions on upload simply set all the sizes to 0 (go to settings > media):