Your photo (site logo) can be uploaded and/or cropped at the Site Information section under Settings. Particular themes may resize the photo depending on the presentation, but the general aspect-ratio is set. To crop a photo to the default shape,  grab the cropping tool handles and adjust. You'll a preview of the cropped photo to the left of the cropping area. You'll want to make sure the original image you upload is at least the size of the default image (180px by 140px).