I’d put it in Portraits. But, I guess it depends on what you want Old Work to represent. If it is meant to provide a venue for understanding how your art has changed and how it has remained the same, then this should go there. If its meant to be a catch-all for things that don’t exactly fit anywhere else, then it should go in Portraits. Either way you might consider a naming convention that includes that year the picture was taken (and/or when the image was created; in case you decide to take an original image and mess with it significantly).