The function isPrintable looks not deep enough into the registry to find the printable program.
Example: Windows 7 x64 JPEG File
In HKEY_CLASSES_ROOT\\.jpg isn't a Subkey "shell\\print\\command", but it points to the Key "jpegfile" which hasn't a "shell\\print\\command", but a "shell\\printto\\command", which is also useful to determine, that a file is printable.
I have the same issue with windows 8.1 using the com interface.
if I print a Jpeg or .bmp from an application to Pdfcreator it works fine, however from c# code the isPrintable function returns false probably due to the registry entry.
we just released PDFCreator 1.7.3 which should fix that. For older versions, you would have to adapt the registry so the print verbs can be found. (but doing the update should be easier ;-))