Srikanth,
I don't think 900 supported ECW so i suggest converting to TIFF if you can. Then add these TIFFs to ArcGIS Explorer (Add Content > Raster Data). once you have added all your rasters, and you have chosen the main basemap you would like then click the ArcGIs Explorer file menu (round button) > Save As > New Basemap. The new basemap will then be available in the Basemap gallery on the Home tab of the ribbon.
Regards
Mike