i am planning to start a pixel advertising site or a image hosting site. which do u think will be profitable.
Neither I expect. Pixel advertising only worked for the first couple of sites - I doubt that any others made more than a couple of dollars. There are free scripts to set up your own site in minutes so there are too many around. Image hosting is a better option but you are competing with so many huge sites that it would be very hard to get your site noticed. The problem with image hosting is the lack of written content to help you rank highly in the SERP's.
From those two I'd go with image hosting. Better find a twist for that image hosting. Offer weekly prizes or something.