Most of the images you use with Media Center Markup Language (MCML) will be in the PNG format if they are part of your <UI> if for no other reason you can embed alpha transparency information within the PNG (which you can't with JPEG or GIF). On a whim I ran the PNG assets we ship with the SDK sampler through PNGOUT and found an average file size savings of just above 50%. This can be a pretty significant size savings for resources in assemblies but can be even more important / significant for web experiences due to bandwidth costs (both in terms of hosting / bandwidth dollars AND perceived responsiveness by the user, Charlie Owen.

You can use these two tools to optimize your PNG files: PNGOUT (Command Line) | PNGGauntlet (GUI)

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