By default, WordPress creates 3 additional versions of any image you upload via the WordPress Media Library. The three sizes by default are:
All three of these sizes are customisable using the Media Settings page at www.yourdomain.co.uk/wp-admin/options-media.php
So, how do I remove default images sizes?
If you want to remove these default images you need to add the snippet below to your functions.php file. The snippet below used the WordPress action intermediate_image_sizes_advanced which unsets the thumbnail, medium, and large default images sizes. All you will have left is the full image size that is uploaded.
As always, before you edit your theme, take a backup. I would also copy and paste your functions.php into a blank text editor tab.
Extra: How do I delete all existing default images?
The above snippet only stops the default images being created on new uploaded images. If you want to delete all the existing thumbnail, medium, and large images that have already been created by default, without going through and deleting them manually, then use the Force Regenerate Thumbnails plugin. Once you have done this, deactivate and remove the plugin.