Image tiddlers, by default, render the image embed in the page. I want it to, instead, link to an image file, to be served as static content by nginx.

The solution is based on the four steps of the official documentation, with a twist.

1. create a copy of the image files on a separate directory

this is achieved via

tiddlywiki PATH-TO-TIDDLYWIKI-FOLDER --savetiddlers [is[image]] DEST-PATH

(note that zsh might have some issues with the square brackets; in my case, I surround [is[image]] in quotes, as follows)

tiddlywiki PATH-TO-TIDDLYWIKI-FOLDER --savetiddlers "[is[image]]" DEST-PATH

This should create a DEST-PATH directory with the image files inside. A sort of backup, for all matters, since the originals are still in your tiddlers folder (by default, PATH-TO-TIDDLYWIKI-FOLDER/tiddlers).

2. edit the canonical URI system tiddler and clear the text field

It is possible to have image tiddlers source content from another URI (the canonical URI), instead of the body of their tiddlers (which is what happens by default). This can be done in an automated way. There's a system tiddler located at $:/core/templates/canonical-uri-external-image. In its contents, one sees

./images/<$view field="title" format="doubleurlencoded"/>

The ./images/ prefix can be changed to anything; the following view widget will append the image file name to it, creating a full path. By substituting ./images with https://sbsbsb.sbs/images, which is the endpoint location of all my static image content in the deploying server, the TiddlyWiki engine will try and load the content from this other URI.

Duplicate the original template tiddler via the TiddlyWiki UI, and change the contents accordingly. I named mine $:/core/templates/sbsbsb-canonical-uri (I'm not sure whether it must necessarily stay in the core, but it works as such), and then run it as such:

tiddlywiki MY-TIDDLYWIKI-PATH --setfield "[is[image]]" _canonical_uri $:/core/templates/sbsbsb-canonical-uri text/plain

One also needs to clear the text field for the image files.

tiddlywiki MY-TIDDLYWIKI-PATH --setfield "[is[image]]" text "" text/plain

And this is it! After doing the usual deployment routine — a combination of the --rendertiddler command

tiddlywiki --rendertiddlers [!is[system]tag[public-eng]] $:/core/templates/static.tiddler.html public text/plain 

which outputs static .html files — the images are externally sourced. It is then a matter of conjuring some proper nginx (or equivalent) configuration to serce them accordingly.