Add a JPG file to your resources and set the build action to "Resource". Then put the following code into the Viewbox. The resource declaration can also be transfered into a resource dictionary.
<Viewbox>
<Grid>
<Grid.Resources>
<BitmapImage
x:Key="image"
UriSource=".\Resources\art.jpg" />
</Grid.Resources>
<Canvas>
<Image
Source="{StaticResource ResourceKey=image}" />
</Canvas>
</Grid>
</Viewbox>