Changing the default header of a phpBB3 forum is easy.
I used this very useful website that is pretty self-explanatory to change the picture shown within the header:
Note: When you refresh your forums to see if the changes have taken place, do so by pressing SHIFT+F5. (This clears the cache and does a full refresh)
A faster way would be to rename your image to “site_logo.gif” and overwrite the default one by moving it into the “styles/*stylename*/imageset/” folder. The image will most likely be scaled to a default size so you must change the size manually by going into Administration Control Panel>Styles>Imagesets>Edit and under Select Image select Main Logo. Select ‘yes’ to Include Dimensions and manually type in the desired image size.
Keep in mind that if your image has a background with a solid colour, it will show up on top of the background of the header, sometimes making it look unprofessional. To fix this you must either create the image with a transparent background, or edit the header background to match the image.
To do the latter, go to Administration Control Panel>Styles>Themes>Edit and comment out the following line by changing:
Right on top of this line, the code “background-color: #FFFFFF;” can be seen. The “FFFFFF” part can be changed to any colour hexidecimal code (more colour codes can be found here).
When done, hit Submit and check out the new forum header (remember to refresh with SHIFT+F5).