You can use RadBitmap to resize your images or pass your WriteableBitmap in the constructor. Then export the RadBitmap with the proper format provider to a stream. Finally, pass this stream to the SetSource method of the BitmapImage you want as a result. Here is a sample code on how to do that.
RadBitmap radImage =
MemoryStream stream =
PngFormatProvider provider =
// Choose the right FormatProvider.
BitmapImage bi =
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