VICTORY! :-)
My javascript toy now shows a link "Add a box" below the image. Click on it, then on the upper left corner of the new-box-to-be in the image, then on the lower right corner. Enter a category name to link to, and you get a ready-to-copy-and-paste template. Just put it in the image description, and voila, new box.
Also, you can now turn existing boxes on and off via a link. A box will become fully transparent when you hover over it with the mouse cursor.
There is no "delete box" function - just edit the description page to remove the appropriate template.
Note: Adding boxes doesn't seem to work in Internet explorer. Probably a minor bug, will fix tomorrow.
Magnus