After a bit of an investigation, it seems like few wallpaper websites had taken Bimal Nepal's picture from his Flickr album and made the picture available for free downlaod in their respective websites.

http://wallpaperpassion.com/download-wallpaper/39898/diwali-diya-wallpaper.html

http://hdwallpapersx.com/diwali-wishes-wallpaper/happy-diwali-2013-greetings-hd-wallpapers/

Now, someone from Modi's staff must have downloaded from those sites thinking it's free stuff.

So it's probobably those websites that need to be answerable.