When you upload an image to the file manager, concrete5 automatically generates various thumbnails of different sizes. This is for use primarily in responsive settings, so that users don't need to download a larger file than they need to for their view port, i.e. a mobile phone. It's also useful for various interfaces within concrete5, i.e. the file manager uses a square thumbnail version to give a quick preview of the image while you are looking at a list of your files. However, you can always edit the thumbnails after they've been generated if you'd like them to appear differently.