/// <summary>
/// 切割一张指定的图片
/// </summary>
/// <param name="source">指定的图片源</param>
/// <param name="tileWidth">图块宽度</param>
/// <param name="tileHeight">图块高度</param>
/// <returns>切割好的图片数组（从左往右，从上往下）</returns>
public static ImageSource[] SplitImage(BitmapSource source, int tileWidth, int tileHeight)
{
    var colCount = source.PixelWidth / tileWidth;
    var rowCount = source.PixelHeight / tileHeight;
    var results = new ImageSource[rowCount * colCount];
    var stride = tileWidth * ((source.Format.BitsPerPixel + 7) / 8);
    var pixelsCount = tileWidth * tileHeight;
    var tileRect = new Int32Rect(0, 0, tileWidth, tileHeight);

for (int row = 0; row < rowCount; row++)
    {
        for (int col = 0; col < colCount; col++)
        {
            var pixels = new int[pixelsCount];
            var copyRect = new Int32Rect(col * tileWidth, row * tileHeight, tileWidth, tileHeight);
            source.CopyPixels(copyRect, pixels, stride, 0);
            var wb = new WriteableBitmap(
                tileWidth,
                tileHeight,
                source.DpiX,
                source.DpiY,
                source.Format,
                source.Palette);
            wb.Lock();
            wb.WritePixels(tileRect, pixels, stride, 0);
            wb.Unlock();
                
            results[row * colCount + col] = wb;
        }
    }

return results;
}

用法：

SplitImage(new BitmapImage(new Uri("E:/Graphics/Characters/a.png")), 70, 101)