During the uploading process, you can choose the thumbnail crop for your image by pressing the "Choose Thumbnail" banner, which can be found at the bottom of your image preview.
Once you have clicked on this, you will be able presented with a screen that allows you to choose a square section of the image to be the thumbnail that will show up when people are browsing the site.
Simply select the area you want, and then click "Use thumbnail".
If you cannot see the thumbnail crop, it may be because you are in the "Basic uploader".
If you can't see an option to use the "Advanced uploader" then this may mean that your browser is needing updated.