You go to System>Config>Replacement messages, and select Extended view at the top right.
Go down to Fortiguard Web Filtering section, select Fortiguard Block page.
You will need to import the logo before you start editing the html code on the right, and to do that you go back to the top of the page, on the left should be " Manage images" , and do a Create new, and import it, with an easy name to manage :)
Note that " Maximum image size is 24 KB"
You most probably will need to try a few to get the size you want, and be able to place it correctly on the page, if you can....
Note also that you can add the URL ans Category with :
<br>
URL: %%URL%%
<br />
Category: %%CATEGORY%%
Hope that helps
Richard