You didn't find that construction
on my web site. What you may have found is
{INCLUDEPICTURE {IF TRUE "{Mergefield Path}{Mergefield Image}.jpg"} \d }
or in your case
{INCLUDEPICTURE {IF TRUE "{Mergefield PhotoPath}Blank.jpg"} \d }
This requires that the image is present at the path set in {Mergefield PhotoPath}. To add the conditional field, that must go outside the construction thus
{ IF {Mergefield PhotoPath} <> "" "{INCLUDEPICTURE {IF TRUE "{Mergefield PhotoPath}Blank.jpg"} \d }" "" }
Mergefield Photopath must return "Q:\\Path\\" without the filename
See also
http://www.gmayor.com/mail_merge_graphics_addin.htm