Custom Network Icon

I know this has been discussed here before.

I’m sorry, I just am still having trouble figuring this out.

I made a custom network for my business’s local Wikipedia page.

sticker ID: davis_wiki
sticker url: http://daviswiki.org/ [:username]
sticker name: DavisWiki

I have an image ready to go named davis_wiki.png

How and where do I upload it to get it into the plug-in default theme?

I am pretty new to this but have been dedicated in teaching myself. For instance, I saw a mention of uploading via FTP, but I do not know what that means or how to find it. But I’d like to learn if someone were willing to give specific directions.

Thanks!

https://wordpress.org/plugins/social-stickers/