如何动态生成WBMP？

<%@ Language=VBScript %>
<%
Function StoB(varstr)
str2bin = ""
For i = 1 To Len(varstr)
varchar = Mid(varstr, i, 1)
str2bin = str2bin & ChrB(AscB(varchar))
Next
StoB = str2bin
End Function
img=chr(0) & chr(0) & chr(5) & chr(2) & chr(0) & chr(&Hf8)

Response.ContentType="image/vnd.wap.wbmp"
Response.BinaryWrite stob(img)
%>