OK, to make it clear, the suggested way when you want to edit a K2 gallery is to delete it and upload the new one. Another way is to configure K2 Media manager to open at the directory K2 saves the galleries. If you do so you will be able to add more images to an existing gallery. This assumes that you know the id of the item in order to find the correct folder. To configure media manager open at media folder go to Content->Media Manager->Options and set the Path to files folder to "media". Keep in mind that even if you resize the image SIG Pro will resize it again regarding it's settings. Finally note that this can only be done in the backend not the front-end.