If you get the asymmetric hd lockscreen it's really simple to change the wallpaper! I'm a noob too but this is really simple. If you know the steps to setup ssh like getting ssh from Cydia on your phone then a desktop client like Cyberduck or Filezilla these are the steps.
Turn on ssh on your phone
Open desktop client
Select sftp
The address for your Phone is 192.168.0.3
Username is ROOT
password is ALPINE
Port is 22
Once you connect go into the /var folder then stash then you will see your themes folder for winter board. Double click the asymmetric hd folder and there is a file called BG.jpg. That is the wallpaper the theme comes with. Delete that and them on your desktop grab your on wallpaper, rename it to BG.jpg and drag it back into where you removed the other wallpaper from.
Then in winterboard on your phone select the asymmetric hd theme and respiring and your done