Well, the lock does show up, despite this. I managed to host the image on the notice locally, so that's all set. Thanks for all the help, Ozzy, and everyone else.
Should not matter for the folder. Try one more thing tonight, rename the image to site-logo. Then add this to the image path in settings, @styles/default/xenforo/site-logo.png
If that don't work I'll have to do some more research tomorrow.
I'm noticing in the error console that two members' signatures are triggering flags as insecure. What is the work-around, if we can't upload a signature image locally?