My thanks again for the advice and suggestions.
This process now appears to work satisfactorily (for me).

1. Import the original graphics file into Inkscape.
2. Use the Menu 'Path/Trace Bitmap' and choose the tracing system required.
3. Export as SVG.
4. Import the SVG into Blender, and apply an Extrusion.
5. Export the object as OBJ (or whatever is required).
6. Import the OBJ into Poser, and use the Material room to replace the default (black) colour.

Inkscape's options for tracing are described in the Helpfile. Depending how it is set, the trace may either either generate only the outline or treat the whole coloured area as a solid.
The Inkscape tracing options are described here:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Trace.html