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Then you can just edit the theme file so that the new images are shown (as well), upload the file and make sure that in your admin general preferences you enable the 'automatically detect new theme files' option (or something like that...). If you want to keep the current theme available as well, then I suggest you make a copy of the current theme folder and rename the copied folder to the new theme name. Edit the files and upload the new theme folder. In your admin panel make the new theme available for selection and set it as the default theme.