I tried changing logo but for some reason it didn't work. Instead of the new logo I just ended up with an image marker. In chat/login/settings.php I replaced the pfclogin.png (line 69) with my own image file name and I placed my png file in the login/img directory.
Try to see if there is any cache going on, test if you can reach the image , that variable hold a regular html code, so it should be working or wouldn't work if the image inst reachable.
regards.