Once you duplicate the theme and rename the new theme, you'll need to change that path to the new themes name. For instance, if you're working with the theme named Coffee, then your path is correct as you stated it: "/uStore/Images/Coffee/app/images/header_slide_1.jpg".
If you duplicate that theme under the appearance tab of uStore, you'll have a new name for the theme (let's call it "Coffee_v2" for simplicity). However, your image path in your newly named theme will still point to the images folder of your original Coffee theme. You will need to edit your store-description-en-US.html file in your new Coffee_v2 theme to point at the images folder used by that theme. In this case, you'd want to change that line of code to: "/uStore/Images/Coffee_v2/app/images/header_slide_1.jpg". Then it will be pulling from the images folder associated with your new theme, instead of from the one you duplicated.
That make sense?
You will also need to change the storeId="xxxxxx" and the product group IDs in the config.xml file, as it essentially duplicates all the info when you duplicate a theme.