In the directory where you have installed phpnuke, find the 'theme' directory. Inside the Theme directory, you will find a number of folders; each of them is a theme that you can choose for your phpnuke. Inside each of these folders, navigate to the 'images' folder and replace the 'logo.gif/jpg' with your own graphic of the same size and background colour so that it seamlessly fits into the overall design.
e.g
http://yourdomain.com/community/them...mages/logo.gif
Once you log in into phpnuke as Admin, you can see the admininstration panel. From preferences, you can set the default theme. There are eight themes preinstalled for you to choose from.
3d-Fantasy, Extralite, Kaput, Nukenews, Slash, Slashocean and Traditional.
Each of these themes are located as I said before inside the themes Directory under Community. And they all have images folder and subsequently logo.gif
If you have difficulty designing them, let me know about logo needs and maybe I can help.