Did that before, tried another route and had no luck... So, this is what I finally did and had success..
Opened var/library/themes/analock HD via WinSCP. Tried all the suggestions with no luck. Un-selected the theme via winterboard, deleted from cydia and then deleted the analock folder from inside the directory via WinSCP. Reinstalled the theme through cydia, this time I noticed the theme did not instal to var/library/themes, but instead iFile says i'ts now located in /var/stash/Themes.cYR05S. From there, I was able to follow your steps listed in the link and edit my CSS file, and upload my wallpaper. Now I had my desired results. I was able to select my own background, and it was in a high resolution. Not sure if this is expected, but just wanted to post up incase anyone else is having any issues... I like this theme so much I was determined not to give up! Thanks again for your help and guidance!