Do you mean create the PictureMarkerSymbol in code-behind or simply access from XAML resource as in this sample:http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#AddGraphicsThese two are equivalent code:
<esri:PictureMarkerSymbol x:Key="GlobePictureSymbol" OffsetX="8" OffsetY="8"
Source="/Assets/images/globe-16x16.png" />
PictureMarkerSymbol pms = new PictureMarkerSymbol()
{
OffsetX = 8,
OffsetY = 8,
Source = new BitmapImage(new Uri("/Assets/images/globe-16x16.png", UriKind.RelativeOrAbsolute))
};