500px displays the preview thumbnails of your images in the centre by default. You can edit the crop by opening your Organizer, hovering over the thumbnail, and clicking on Edit.
From the edit screens, you can simply click on the image and drag to create a square crop. You can adjust the crop or move it around the image until you are happy with the new preview.
Don't forget to click Save when you are done.
Please note, the updated thumbnail may take up to 20 minutes to display. This is due to the thumbnail images being cached across many servers, as well as on your local computer. Be patient and try viewing the thumbnail after approximately 20 minutes to see the updated image.
Need more help? Email firstname.lastname@example.org or tweet @500px_help