take battery and sd card off.
leave it off for 3 minutes.
after 3 minutes put the sd card and battery and turn device on.
see if this fixed the problem. if not go to settings and theres a unmount sd card button click that then turn phone off remove battery then sd card then re-insert the sd card and battery power on.
see if the device ask you if you want to format, do a backup then format. you should be good to go.
now try downloading your wallpaper.
however if it doesn't tell you to format the sd card, just skip that part.
goodluck and let is know how it goes.
English is my second Language