I have an image of size 144 pixels (I measured the image size using both irfanView and Photoshop).
However, when opened in a window using the following xaml, the image's width, measured both by the ActualWidth parameter and by the same graphic software, is 192 pixels.
It turns out that 141 points = 192 pixels (as 1pt=1.33px).
So it seems like .Net opens the image, measure it size in pixels and draws the image with the same size but in points.
Here is the XAML and code:
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