Adding image in header in appian By using HTMl

I’m configuring the logo in the header of my Appian HTML code by using a constant value that holds the image. In the <img> tag, I am specifying the source as this constant value.

<img logoId="&document(cons!EX_LOGO,"id")&">
but it is not displaying image in pdf.
Please give me any suggesstions,
Thanks.