不实例化图片，获取图片宽高的方法（vb.net）

'''<summary>
    ''' 不打开图片获取图片大小
    '''</summary>
    '''<param name="filename"></param>
    '''<param name="w"></param>
    '''<param name="h"></param>
    '''<remarks>http://bbs.pfan.cn/post-323575.html</remarks>
    Public Shared Sub GetImageSize(ByVal filename As String, <Out()> ByRef w As Integer, <Out()> ByRef h As Integer)
        Dim k As Long, H1 As Byte, H2 As Byte, W1 As Byte, W2 As Byte
        Dim binReader As New BinaryReader(File.Open(filename, FileMode.Open))
        H1 = binReader.ReadByte()
        H2 = binReader.ReadByte()
        W1 = binReader.ReadByte()
        W2 = binReader.ReadByte()

        If H1 = 255 AndAlso H2 = 216 Then 'jpg
            k = 3
            Do
                binReader.BaseStream.Seek(k - 1, SeekOrigin.Begin)
                H1 = binReader.ReadByte()
                H2 = binReader.ReadByte()
                W1 = binReader.ReadByte()
                W2 = binReader.ReadByte()

                k = k + 2 + W2 + W1 * 256
            Loop Until H1 = 255 And H2 = 192
            H1 = binReader.ReadByte() '每个样本位数,舍弃
            H1 = binReader.ReadByte()
            H2 = binReader.ReadByte()
            W1 = binReader.ReadByte()
            W2 = binReader.ReadByte()
        ElseIf H1 = 0 AndAlso H2 = 0 AndAlso W1 = 1 Then 'icon
            binReader.BaseStream.Seek(7 - 1, SeekOrigin.Begin)
            W2 = binReader.ReadByte()
            H2 = binReader.ReadByte()
            W1 = 0 : H1 = 0
        ElseIf H1 = 66 AndAlso H2 = 77 Then 'bmp
            binReader.BaseStream.Seek(19 - 1, SeekOrigin.Begin)
            W2 = binReader.ReadByte()
            W1 = binReader.ReadByte()
            binReader.BaseStream.Seek(23 - 1, SeekOrigin.Begin)
            H2 = binReader.ReadByte()
            H1 = binReader.ReadByte()
        ElseIf H1 = 71 AndAlso H2 = 73 AndAlso W1 = 70 Then 'gif
            binReader.BaseStream.Seek(7 - 1, SeekOrigin.Begin)
            W2 = binReader.ReadByte()
            W1 = binReader.ReadByte()
            H2 = binReader.ReadByte()
            H1 = binReader.ReadByte()
        ElseIf H1 = 137 AndAlso H2 = 80 AndAlso W1 = 78 AndAlso W2 = 71 Then 'png
            binReader.BaseStream.Seek(19 - 1, SeekOrigin.Begin)
            W1 = binReader.ReadByte()
            W2 = binReader.ReadByte()
            binReader.BaseStream.Seek(23 - 1, SeekOrigin.Begin)
            H1 = binReader.ReadByte()
            H2 = binReader.ReadByte()
        ElseIf H1 = 215 AndAlso H2 = 205 AndAlso W1 = 198 AndAlso W2 = 154 Then 'wmf
            Dim L As Integer, R As Integer, T As Integer, B As Integer
            binReader.BaseStream.Seek(7 - 1, SeekOrigin.Begin)
            L = binReader.ReadInt32
            T = binReader.ReadInt32()
            R = binReader.ReadInt32()
            B = binReader.ReadInt32()
            L = R - L : T = B - T
            W2 = L Mod 256 : W1 = L \ 256 : H2 = T Mod 256 : H1 = T \ 256
        ElseIf (H1 = 77 AndAlso H2 = 77) OrElse (H1 = 73 AndAlso H2 = 73) Then 'tif
            Dim de As Integer, t1 As Integer, t2 As Long
            t2 = binReader.ReadInt64()      '获取第一个IFD的偏移量

            binReader.BaseStream.Seek(t2, SeekOrigin.Begin) '获取第一个IFD中DE的数量

            For i = 1 To de
                t1 = binReader.ReadInt32 '获取属性的标签编号

                If t1 = 256 Then     '如果是图像宽
                    t1 = binReader.ReadInt32 '废弃
                    t2 = binReader.ReadInt64 '废弃
                    W2 = binReader.ReadByte
                    W1 = binReader.ReadByte
                    t1 = binReader.ReadInt32 '废弃

                    k = k + 1
                ElseIf t1 = 257 Then '如果是图像高
                    t1 = binReader.ReadInt32 '废弃
                    t2 = binReader.ReadInt64 '废弃
                    H2 = binReader.ReadByte
                    H1 = binReader.ReadByte
                    t1 = binReader.ReadInt32 '废弃
                    k = k + 1
                Else                 '否则这些数据废弃
                    t1 = binReader.ReadInt32
                    t2 = binReader.ReadInt64
                    t2 = binReader.ReadInt64
                End If
                If k = 2 Then Exit For
            Next i
        Else
            Exit Sub
        End If

        w = W1 * 256 + W2
        h = H1 * 256 + H2

    End Sub

参考：http://bbs.pfan.cn/post-323575.html

只有jpg和png测试过，其他不能保证正确，可以参考原文改进。