Why does the picture on my camera monitor look grainy

i bought a new monitor for our 4 camera surveilance and the pictures is more grainy than the one that broke down. that one had 2 bnc cable hookups. I was told not to spend money to replace those but to go to store and buy any tv monitor and get a bnc adaptor. well i went and got a 720p 19 inch dynex from best buy for 160.00 but after hooking up and tuninig it, the pic looks grainy where the colors are bleeding into other colors. the rep said that with high def monitors you need to get high def feeds, but our cameras are just digital not high def. Any one got solution. also my original monitor an AGN-Pro has gotten darker after i unplugged one of the bnc cables last week.It is hard to see screen is there a remedy for that.