After uploading your photos or simply dragging them into the "What Does It Look Like?" section of the listing form, click "Edit" underneath the thumbnail version of the photo you'd like to edit. From there, just crop the photo as you see fit, then click "Done". You can rotate your photos in 90-degree increments using the circular rotate button. When you've finished cropping and rotating that photo, click "Save".
Currently, you'll only have the ability to crop your photos into a square shape.