Greetings, Valérian,

If you run the report that has the logo on it via the "Screen" option, you will be able to see the report like any other page of Sage CRM. In that format, a right-click, "Inspect Element" of the thing you're curious about changing or investigating will allow you to  gain more insight.

In the below screen snip, I've right-clicked the logo, "Inspect Element"-ed, and I can see that the path to the logo file is located at "http://[servername]/[instance]/reports/logo.jpg" - which translates to where your Sage CRM application files are installed and in the "WWWRoot/reports/" directory.


Below is what that same location maps to on my local install of Sage CRM:


Notice that there are two logo files (pdflogo.jpg and Logo.jpg) - be sure to replace both accordingly to ensure the correct file gets associated with the output of the report you've selected.

Hopefully that does the trick! 

There are 2 places: in the CRM install folder/WWWRoot/Reports and the files are called pdflogo.jpg and Logo.jpg. Keep backups because if you upgrade, they will get overwritten.