The first sequence you performed created a raster image, not a vector image. You need to use an intermediate program to convert the scan to vector. A great free program for that purpose, one I've used often, is Inkscape. The Trace Bitmap feature will present a few options. For beginning conversions, leave the defaults as is.

If you want to go a bit farther, grab Paths to OpenSCAD extension for Inkscape, and OpenSCAD, of course. Once you convert to vector, you can generate OpenSCAD code and create an STL directly from OpenSCAD.

You'll get more accurate results with your scanner. If you don't need precision, even a photograph can be used.