How to change or remove the top logo in phpBB3
You can change or remove the default top logo of your phpBB3 forum from the administration control panel.
After you log in to your admin panel click on the Styles tab and then on the Imagesets link, which is on the left side of the Styles page. Then click on the Edit button for the particular style.
On the Edit imageset page make sure that the Select image drop-down menu is set to Main logo and from the Image drop-down menu either select No image to remove the logo completely, or, if you want to change it, select another image that you have previously uploaded.
If you want to change the image with your own logo, first you have to upload it to the imageset folder for the particular style.
When it comes to removing, you can also remove the logo by commenting out some code in the overall-header.html file of style.
For some more details you can also check out the tutorial on changing or removing the top logo in phpBB3.