Gents, and WPGR peeps at large,
I figured out what the issue was... it is a recent upgrade to the server's nginx config. I noted that when I right-clicked on images that were missing to see what the error was -- sure enough, the error included "nginx" in its verbiage. I confirmed that with Topher when I ran it past him and he said "Oh! I had that happen before too on another setup for multisite." :)
So, thanks for the responses -- I think we're good now. David, MiAR is getting some facelifts this year... so there will be even more to love about Mi. Adventure Racing!