How come i cant change my lock background on my iphone?

I jailbroke it, and orginally i could change my lock background...now I cant seem to change it anymore it just stays on one pic...i have tried going in the settings and changing the wallpaper but it still doesnt work!?
How come i cant change my lock background on my iphone?
I think you should change the theme of your iphone.

row cell

fish