It depends what you mean by crop. If you really mean resize, then just set the size of the img element. If you actually mean to extract a portion of the image, then you can do it before the image is rendered. Something like this;
Bitmap bmp = new Bitmap(existingImage);
Bitmap bmpCrop = bmpImage.Clone(croppedArea, bmp.PixelFormat);
return (Image)(bmpCrop);