Hi. I am wanting to change existing pictures on my site via FTP. I just download them (the pictures in the src folder), edit, and reupload. My problem is this: how do I clear out the images in the cache folder to reflect the new images?

If you are updating the item images, they must be uploaded by the k2 backend item add or edit...they are generated when you save the k2 item and stored with different filenames.