Because an apple is the perfect logo to represent a company
named Apple.
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Below
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The apple logo.
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The apple is the logo for the Macintosh company that made the
iPod.
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An administrator can go to Special:ThemeDesigner (it'll be on your tools in the toolbar too) and upload a logo in the Wordmark tab. The logo must be 150x65px, and as I recall must be a .PNG image.