You can click the Image Management button on the product you would like to edit images for.
You will see the following screen. You can drag and drop images on to the upload box from any folder on your computer.
Once you have uploaded your product images, you can sort them so they display in the order you would like. The first image will be the featured image on the product listing page as well as product details page. You can drag it to a new location when you hover over the image and the cursor changes to the four arrow pointer.
Deleting an Image
To delete an image, click on the delete button on the image icon:
To crop an image within your store, click on the crop icon on the thumbnail:
Use the grid to select the portion of the image you would like to keep. Then, hit the "Crop" button: