The setup we have is for multiple journals, so we want to change the default favicon in OJS3, however we want to use a PNG file instead of the ICO file. Where can we find the directive to change the default favicon?
Thank you so much for any help! 
Hmmm… I think that there is currently no such place, for the favicon for the index site, but I will double check… – For journals you would use the favicon upload possibility on the page Settings > Website > Appearance.
Maybe to use .ico and upload it in the OJS folder, so that apache handles it?
Best,
Bozana
I think that you can upload the default favicon to the installation root: https://github.com/pkp/ojs/blob/master/favicon.ico
But I do not know how it would support png.
Thanks @bozana and @ajnyga! I do have access to a favicon in ico format. I actually wanted to try using PNG, because the files are usually smaller and easier to manage. I will look into how Apache calls the favicon. It never struck me that it was not controlled by PHP! 
However, I am aware of the ability to upload the same for individual journals. If I have any luck I will definitely keep you posted.