终于找到了，谢谢大家

<%
Public Function getImgW(IMGPath, maxW, maxH)
Dim scale, scale1, scale2
Dim imgW1, imgH1
imgW1 = imgW(IMGPath)
imgH1 = imgH(IMGPath)
scale1 = imgW1 / maxW
scale2 = imgH1 / maxH
if scale1 > scale2 then
scale = scale1
else
scale = scale2
end if
getImgW = CINT(imgW1 / scale)
End Function

Public Function getImgH(IMGPath, maxW, maxH)
Dim scale, scale1, scale2
Dim imgW1, imgH1
imgW1 = imgW(IMGPath)
imgH1 = imgH(IMGPath)
scale1 = imgW1 / maxW
scale2 = imgH1 / maxH
if scale1 > scale2 then
scale = scale1
else
scale = scale2
end if
getImgH = CINT(imgH1 / scale)
End Function
Class ImgWHInfo '»ñȡͼƬ¿í¶ÈºÍ¸ß¶ÈµÄÀ֧࣬³ÖJPG£¬GIF£¬PNG£¬BMP
Dim ASO
Private Sub Class_Initialize
Set ASO=Server.CreateObject("ADODB.Stream")
ASO.Mode=3
ASO.Type=1
ASO.Open
End Sub
Private Sub Class_Terminate
Err.Clear
Set ASO=Nothing
End Sub

Private Function Bin2Str(Bin)
Dim I, Str
For I=1 To LenB(Bin)
clow=MidB(Bin,I,1)
If ASCB(clow)<128 Then
Str = Str & Chr(ASCB(clow))
Else
I=I+1
If I <= LenB(Bin) Then Str = Str & Chr(ASCW(MidB(Bin,I,1)&clow))
End If
Next
Bin2Str = Str
End Function

Private Function Num2Str(Num,Base,Lens)
Dim Ret
Ret = ""
While(Num>=Base)
Ret = (Num Mod Base) & Ret
Num = (Num - Num Mod Base)/Base
Wend
Num2Str = Right(String(Lens,"0") & Num & Ret,Lens)
End Function

Private Function Str2Num(Str,Base)
Dim Ret,I
Ret = 0
For I=1 To Len(Str)
Ret = Ret *base + Cint(Mid(Str,I,1))
Next
Str2Num=Ret
End Function

Private Function BinVal(Bin)
Dim Ret,I
Ret = 0
For I = LenB(Bin) To 1 Step -1
Ret = Ret *256 + AscB(MidB(Bin,I,1))
Next
BinVal=Ret
End Function

Private Function BinVal2(Bin)
Dim Ret,I
Ret = 0
For I = 1 To LenB(Bin)
Ret = Ret *256 + AscB(MidB(Bin,I,1))
Next
BinVal2=Ret
End Function

Private Function GetImageSize(filespec)
Dim bFlag
Dim Ret(3)
ASO.LoadFromFile(filespec)
bFlag=ASO.Read(3)
Select Case Hex(binVal(bFlag))
Case "4E5089":
ASO.Read(15)
ret(0)="PNG"
ret(1)=BinVal2(ASO.Read(2))
ASO.Read(2)
ret(2)=BinVal2(ASO.Read(2))
Case "464947":
ASO.read(3)
ret(0)="gif"
ret(1)=BinVal(ASO.Read(2))
ret(2)=BinVal(ASO.Read(2))
Case "535746":
ASO.read(5)
binData=ASO.Read(1)
sConv=Num2Str(ascb(binData),2 ,8)
nBits=Str2Num(left(sConv,5),2)
sConv=mid(sConv,6)
While(len(sConv)<nBits*4)
binData=ASO.Read(1)
sConv=sConv&Num2Str(AscB(binData),2 ,8)
Wend
ret(0)="SWF"
ret(1)=Int(Abs(Str2Num(Mid(sConv,1*nBits+1,nBits),2)-Str2Num(Mid(sConv,0*nBits+1,nBits),2))/20)
ret(2)=Int(Abs(Str2Num(Mid(sConv,3*nBits+1,nBits),2)-Str2Num(Mid(sConv,2*nBits+1,nBits),2))/20)
Case "FFD8FF":
Do
Do: p1=binVal(ASO.Read(1)): Loop While p1=255 And Not ASO.EOS
If p1>191 And p1<196 Then Exit Do Else ASO.read(binval2(ASO.Read(2))-2)
Do:p1=binVal(ASO.Read(1)):Loop While p1<255 And Not ASO.EOS
Loop While True
ASO.Read(3)
ret(0)="JPG"
ret(2)=binval2(ASO.Read(2))
ret(1)=binval2(ASO.Read(2))
Case Else:
If left(Bin2Str(bFlag),2)="BM" Then
ASO.Read(15)
ret(0)="BMP"
ret(1)=binval(ASO.Read(4))
ret(2)=binval(ASO.Read(4))
Else
ret(0)=""
End If
End Select
ret(3)="width=""" & ret(1) &""" height=""" & ret(2) &""""
getimagesize=ret
End Function

Public Function imgW(IMGPath)
Dim FSO,IMGFile,FileExt,Arr
Set FSO = Server.CreateObject("Scripting.FileSystemObject")
If (FSO.FileExists(IMGPath)) Then
Set IMGFile = FSO.GetFile(IMGPath)
FileExt=FSO.GetExtensionName(IMGPath)
Select Case FileExt
Case "gif","bmp","jpg","png":
Arr=GetImageSize(IMGFile.Path)
imgW = Arr(1)
End Select
Set IMGFile=Nothing
Else
imgW = 0
End If
Set FSO=Nothing
End Function

Public Function imgH(IMGPath)
Dim FSO,IMGFile,FileExt,Arr
Set FSO = server.CreateObject("Scripting.FileSystemObject")
If (FSO.FileExists(IMGPath)) Then
Set IMGFile = FSO.GetFile(IMGPath)
FileExt=FSO.GetExtensionName(IMGPath)
Select Case FileExt
Case "gif","bmp","jpg","png":
Arr=getImageSize(IMGFile.Path)
imgH = Arr(2)
End Select
Set IMGFile=Nothing
Else
imgH = 0
End If
Set FSO=Nothing
End Function
Public Function getImgW(IMGPath, maxW, maxH)
Dim scale, scale1, scale2
Dim imgW1, imgH1
imgW1 = imgW(IMGPath)
imgH1 = imgH(IMGPath)
scale1 = imgW1 / maxW
scale2 = imgH1 / maxH
if scale1 > scale2 then
scale = scale1
else
scale = scale2
end if
getImgW = CINT(imgW1 / scale)
End Function

Public Function getImgH(IMGPath, maxW, maxH)
Dim scale, scale1, scale2
Dim imgW1, imgH1
imgW1 = imgW(IMGPath)
imgH1 = imgH(IMGPath)
scale1 = imgW1 / maxW
scale2 = imgH1 / maxH
if scale1 > scale2 then
scale = scale1
else
scale = scale2
end if
getImgH = CINT(imgH1 / scale)
End Function
End Class
%>

<%Set PP=New ImgWHInfo %>
<img src=<%=Trim(Rst(0))%> width=<%=PP.getImgW(Server.Mappath(Trim(Rst(0))),500,500)%> height=<%=PP.getImgH(Server.Mappath(Trim(Rst(0))),500,500)%> border="0" />
<%Set pp=Nothing %>