Is there any way/API available in order to change the home/lock screen background wallpaper programmatically?
There is no API that provides this functionality at this time. There is already a feature request for this on UserVoice that you can vote on here: http://wpdev.uservoice.com/forums/110705-app-platform/suggestions/1720049-provide-a-wallpaper-api-to-enable-in-app-setting-o?ref=title