Keep in mind that those instructions assume that you have tiddlywiki on node.js installed globally.
Here's a script/command line example:
tiddlywiki --load tw-with-images.html \
--output test4 \
--savetiddlers [is[image]] images \
--setfield [is[image]] "_canonical_uri" \
"$:/core/templates/canonical-uri-external-image" \
"text/plain" \
--setfield "[is[image]]" "text" "" "text/plain" \
--rendertiddler "$:/core/save/all" "tw-external.html" "text/plain"
where tw-with-images.html is your source file, "images" is the name of the image directory, test4 is a directory where the output (images and html) will go, and tw-external.html is the new TW file where images have been replaced with external image links.
The caveat is that I think you can only run this once against your source file. I'm not sure what will happen with to images that have already been replaced with external uris. Likewise, double check any of your source image tiddlers that you have already replaced by hand with _canonical_uri paths. I'm sure someone smarter than me could improve the filtering in the above so that it skips images that use the _canonical_uri field.
Be sure to make backups before trying this!
Good luck!