Typically you can overlay vector on raster but not raster on vector.
Vector is any of points, lines, or polygons. Raster is of course pixels.
I'm not familiar with IDRISI but in ArcMap you can overlay vector on raster. It appears you can do the same with IDRISI.
The country boundaries are vector; the underlying image is raster.
Maybe you need to reproject your vector data to match the raster data?