The easiest way is to activate lazy loading. Then your images will only load when you specifically open an image tiddler.
Other approaches are to externalize the files.
If you put files into a subdirectory called “files”, then node.js will serve up your files (images) from that subdirectory or its subdirectories. You can reference them on a relative path using an image link or the image widget. You can also create image tiddlers by setting the “type” field of your tiddler to the type of image (e.g. image/jpg) and the _canonical_uri field to the path of the file.
To save your image to the local directory you can either do the usual right-click and save-as thing, or you can export the image.
Because creating _canonical_uri tiddlers can be a bit tedious, you can also load your TW using TiddlyDesktop and use the official external files plugin. This will allow you to drag images into your TW file and have the image tiddler made for you automatically.
Another approach, if you convert your node.js TW into a single file, is to use Saq’s File Upload plugin. However this requires you to serve up your TW file via a webdav server. This can be done conveniently with Rclone (see rclone.org).