Hi
This is probably something you should take up with FreePBX, however, they may feel that replacing their logo's is tantamount to "passing off"
They do make provision for the logo to be changed on the top right within amportal.conf, but no provision for the logo on the top left, so maybe they don't intend you to change it.
In any case, any changes to the logo on the left are likely to overwritten by a freePBX framework upgrade at some later stage.
Addtionally, in later versions of PiaF, the instructions for the change of logo have changed slightly.
Create a logo called custom-logo.png, 55 pixels high, and replace the existing custom-logo.png in /var/www/html/welcome and /var/www/html/admin/images
If you have a light coloured background in your logo, it often looks better in FreePBX if you make the background transparent.
Yours
Joe