How can we drape a simple jpeg image which doesnot have any geo referencing information over the digital elevation model in order to visualize it in 3D in QGIS?
QGIS now has a very impressive rendering engine which you can use to drape one image over another. For example you can get a nice "3D effect" by draping an ortho-photo over a hillshade raster. You set the rendering in the first raster's Layer Properties page on the "Style" tab. There are several kinds of rendering, the "Darken" or "Multiply" options gives an excellent 3D effect.
To use this rendering you'll need to do two steps in advance: first geo-reference your jpeg image, and second, use your digital elevation model to create a hillshade raster. Both of these procedures are in the Raster menu in QGIS.
Try the GDAL tools plugin. You could try georeferencing your image, it would probably be as a tile that might be a targa image, conversions you may be able to make in photoshop, If you just want a visual or a VRML out, viewable in a browser display with vrml viewer extensions installed you can try "http://www.hangsim.com/3dem/" 3DEM freeware on XP
You can use GRASS GIS for it (perhaps even via GRASS toolbox of Sextante in QGIS), it offers 3D visualization through its nviz module. For examples, see: http://grass.osgeo.org/screenshots/3D (e.g., Perspective Landsat TM RGB visualization).