Create a new document 400 pixels square. It can be RGB or CMYK.
Place a photo into the document. For this illustration, I'm using a tightly-cropped square portrait. Uncheck the Link button, in the Place dialog, so the photo will be embedded. If you forget this step, simply go to the Links panel and embed the image.
Make sure the photo is 400 pixels by 400 pixels. If necessary, you can resize it in the Control Panel. The beauty of this technique is that you can start with a less-than-perfect photo and get a nice result.