In case someone else stumbles across this problem:
I could not find this documented anywhere, but what seems to work is to just copy the image in the tiddlers directory, then restart the nodejs server, and search for the image title from tiddlywiki. There will be a tiddler that contains that image, that you can edit at your leisure.
Alternatively, copy the image as image_name.png (or image_name.jpg) into the tiddlers directory, and create a image_name.png.meta text file with the following contents:
title: image_name
type: image/jpeg
Upon restart of the tiddlywiki nodejs server, a tiddler with title image_name which contains the image will be there.