GeometryModel3D model = .;
string imgFile = @"C:\a.jpg";
Uri imgUri = new Uri( imgFile, UriKind.RelativeOrAbsolute );
ImageSource imgSrc = new BitmapImage( imgUri ); //load image
DiffuseMaterial mtr = new DiffuseMaterial();
mtr.Brush = new ImageBrush( imgSrc );
model.Material = mtr;
string imgFile = @"C:\a.jpg";
Uri imgUri = new Uri( imgFile, UriKind.RelativeOrAbsolute );
ImageSource imgSrc = new BitmapImage( imgUri ); //load image
DiffuseMaterial mtr = new DiffuseMaterial();
mtr.Brush = new ImageBrush( imgSrc );
model.Material = mtr;