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Then find your skin, click on the "+" to expand it, and click "Edit start.html":
You'll now see the file's contents to the right.
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The HTML code should now look like this:
Save the start.html file, then reload a Genie page, and you should see the logo you uploaded.
That's it!
Note that if your logo is a JPG file you should have saved the uploaded image as logo.jpg, and used this HTML tag instead:
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