My knowledge of Varnish is very limited - but don't you have to either pipe Varnish into nginx and then NodeBB or nginx into Varnish and then NodeBB? From your configuration, it seems you're trying to pipe nginx into NodeBB on port 2000 and the same for Varnish. Can Varnish and nginx coexist on the same port?
Here is my working configuration but against, I'm running nginx to Varnish's default port (e.g., 6081).
@hollosch rather than dimension, I believe image/writing itself is more important. You can move/adjust the picture after you upload it; however how it looks on the desktop will be different from how it looks on mobile.