Hi there,
You can go to your manage images tab and select the photo and click on edit. From there, you have the option to resize, rotate, or crop. You can also access this editing feature when you are making your listing. There are 2 links under the photo - one for edit, the other for remove.
Accessing the edit feature from the manage images folder is better if you have several in a row that you want to edit, because you can go to the previous and next photos without going back to the manage images screen.
Hope this helps!