A simple way would be to just replace the phpbb image file with your own with the same dimensions.
Well, after you replace the image file you'll have to go to the admin control panel and follow these steps:
1. Go to the "Styles" tab.
2. On the left hand side under "Style components", click the first link under it.
3. On the right under "Actions" find the template you are using (prosilver if you're not using a different template, and it's the standard blueish theme) and click "Refresh"
4. Repeat steps 2-3 for the other two links.
The image should have then changed to your image.
