In one of his blog post (here or here, for the French version), the Windows 8 Development Master David Catuhe explained how to generate and to rescale a new picture for a tile (secondary or not).

This is very useful if you're not able to modify the service delivering the images to rescale them automatically. Its technique need you to use an Image HTML element and, unfortunately, you're not allowed to use graphical components in a Windows 8 background task.

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I guess you came to this post by searching similar kind of issues in any of the search engine and hope that this resolved your problem. If you find this tips useful, just drop a line below and share the link to others and who knows they might find it useful too. 

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