First, I recommend you read https://en.wikipedia.org/wiki/WebP#Criticism . Secondly, since WebP is the web standard for images, you can try using WebBrowser as your image control. Finally, I believe that the reason you cannot reduce the quality is because you want to use the same image as the thumbnail. I suggest you create a process that will generate a thumbnail, for example, 480px x 800px. Now your Windows Phone application can display a “preview” of the image, and if the user wants to see the full one, he must wait.
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