Look I don't want to cut your enthusiasm and stuff, but it more seems like spamming to me.
man if you are feeling it is a little complex then just simply back up the orginal files then get your new backgrounds and name them as the orginals and dont forget to get them as 800×480 png files then once you have your new backgrounds correctly as they needed use cp -f to copy and overwrite the orginal png files and again dont forget to back up the orginal backgrounds in case you missed up something and thanx for your reply