Constant or variable?

[Blobby]

The following piece of code blurs a picture box but the compiler wont let me use a variable for the picture width or height, it only wants a constant. Can anyone tell me Why i cant use a variable (intUpperBoundY) in the DIM PIXELS line (1st line of code)? This value may change depending on the size of the picture I am blurring and so cant use a constant. Thanks for any pointers

Private Sub Blur(intUpperBoundX, intUpperBoundY)
    Dim Pixels(1 To intUpperBoundX, 1 To intUpperBoundY) As Long
    Dim x, y As Integer
    Dim bytRed, bytGreen, bytBlue As Byte
    
    For x = 1 To intUpperBoundX
        For y = 1 To intUpperBoundY
            Pixels(x, y) = Picture1.Point(x, y)
        Next y
    Next x
    
    For x = 1 To intUpperBoundX - 1
        For y = 1 To intUpperBoundY
            bytRed = Abs((Pixels(x + 1, y) And &HFF) + (Pixels(x, y) And &HFF)) / 2
            bytGreen = Abs(((Pixels(x + 1, y) And &HFF00) / &H100) Mod &H100 + ((Pixels(x, y) And &HFF00) / &H100) Mod &H100) / 2
            bytBlue = Abs(((Pixels(x + 1, y) And &HFF0000) / &H10000) Mod &H100 + ((Pixels(x, y) And &HFF0000) / &H10000) Mod &H100) / 2
            Pixels(x, y) = RGB(bytRed, bytGreen, bytBlue)
         Next y
    Next x
    
    
    For x = 1 To intUpperBoundX
        For y = 1 To intUpperBoundY
            Picture2.PSet (x, y), Pixels(x, y)
        Next y
    Next x
    
End Sub

Is it possible to create a constant on the fly based on a variable?