Product:
Cognos Analytics 11.0.7
Microsoft Windows 2016 Standard server

Problem:
No picture shown in the PDF report, but they show nice in a HTML report.

Solution:
If you have SSO with IIS, then you need to set Anonymous Authentication on the folders where the images are stored.
This can be more than one folder, depending on where you have your company reports pictures stored.
The sample pictures are stored in folder c:\Program Files\ibm\cognos\analytics\webcontent\bi\samples\images

So you need to start IIS Manager
Expand to your IBMCOGNOS folder
Expand down to bi/samples
Mark images folder and click on Authentication icon
Mark Anonymous Authentication and click on Enable text, to change status to Enabled
Exit IIS
Try to run the report as PDF again.

You can also ensure that the account running the Cognos BI services have full rights to the images folder in NTFS file system.

More Information:
http://www-01.ibm.com/support/docview.wss?uid=swg21656263
http://www-01.ibm.com/support/docview.wss?uid=swg21339267