I know that there is a way to make a transparent window. I have tried with
BitBlt to the desktop window, but it is too slow. I can see how it clears
the image and draws the new one, even with a size of 100x100 pixels!

Concerning transparency BitBlt is correct approach
Try to check code - single BitBlt for the whole screen should take
tens ms depending on graphic card, graphic mode, graphic driver,
CPU speed. There are a lot of examples in www

Concerning shifts - there is nothing in VB. i WOULD
propose to move shifts in C DLL.-
You can use multiply and divide (there is integer division in VB)
and to avoid overflow you should first check the range of results
function shift8left(a as integer)
dim temp as integer
temp = a and 255
if temp >127 then
    temp = temp and 127
    shift8left = (temp*256) or &h8000
else
    shift8left = temp*256
endif
end function
Best regards

Function SHR(Bits As Long, Shift As Long) As Long
    If Shift = 31 Then
        SHR = Bits \ &H80000000
    Else
        SHR = Bits \ 2 ^ Shift
    End If
End Function

If you're dealing with bytes or integers, you then have to cut off the high
bytes.  For example:
Dim X As Byte
X = 101
X = SHL(X, 4) And &HFF&

Dim Y As Integer
Y = 10101
Y = SHL(Y, 8) And &HFFFF&

I actually have several variants of these functions which are faster for
some things and some that do more error checking, etc.  In some cases, you
can get a speed improvement by precalculating the shifts and storing them in
an array...

Just drop the control on your page and it becomes 'translucent' or
transparent. It's a lot easier than doing APIs. Of course this only
works with Windows 2000, and just looks normal everywhere else.

You can also do odd shaped forms with it.