Santosh.

Pretty easy: create an extrafield (type image) and insert different background image for each item. Than, use BNR Content for K2 (extensions.joomla.org/extension/bnr-content-for-k2) to call the specific extrafield in a module position (you gonna need to know how to call extrafields in code, theres a lot of topics teaching how to do that, and how to use templates - the above type of module accept templates, so its easy). Put this module position in the background (you can use fixed position for the background and relative for the content).

I use this in my website, check here differents backgrounds:
www.cafecomfilme.com.br/filmes/o-sono-da-morte
www.cafecomfilme.com.br/filmes/caes-de-guerra

Hope this can help.