ciao ho fatto una ricerca, ma non l'ho provato.
qui usa il FileSystemObject

codice:
Gif files
<%
   Set fso=Server.CreateObject("Scripting.FileSystemObject")
   Set ts = fso.OpenTextFile(Server.MapPath("sample.gif"))
   ts.Skip(6)
   s = ts.Read(4)
   w = asc(mid(s,2,1))*256 + asc(mid(s,1,1))
   h = asc(mid(s,4,1))*256 + asc(mid(s,3,1))
   response.write(w)
   response.write(" ")
   response.write(h)
   ts.close
   set ts = Nothing
   set fso = Nothing
%>

BMP PNG
   
<%
   Set fso=Server.CreateObject("Scripting.FileSystemObject")
   Set ts = fso.OpenTextFile(Server.MapPath("sample.bmp"))
   ts.Skip(18)
   s = ts.Read(6)
   w = asc(mid(s,2,1))*256 + asc(mid(s,1,1))
   h = asc(mid(s,6,1))*256 + asc(mid(s,5,1))
   response.write(w)
   response.write(" ")
   response.write(h)
   ts.close
   set ts = Nothing
   set fso = Nothing
   %>
   
JPEG
   <%
   Set fso=Server.CreateObject("Scripting.FileSystemObject")
   Set ts = fso.OpenTextFile(Server.MapPath("sample.jpg"))
   ts.Skip(2)
   do
    do
    s = ts.Read(1)
    if asc(s) <> 255 then exit do
    loop
    if asc(s) < 192 or 195 < asc(s) then
    s = ts.Read(2)
    l = CLng(asc(mid(s,1,1)))*256 + asc(mid(s,2,1))
    ts.Skip(l - 2)
    else
    exit do
    end if
   loop
   s = ts.Read(7)
   w = asc(mid(s,6,1))*256 + asc(mid(s,7,1))
   h = asc(mid(s,4,1))*256 + asc(mid(s,5,1))
   response.write(w)
   response.write(" ")
   response.write(h)
   ts.close
   set ts = Nothing
   set fso = Nothing
   %>