Hey
After hours of trying I found a solution for how to set a user background in SlantedLock.
You probably know, that in the latest Versions of iOS there is no LockBackground.jpg anymore, there's another file instead. One solution was to link to "Lockbackgroundthumbanil.jpg" which is a low-resolution picture.
My method lets you set up a high-resolution picture.
At first you need three apps and anything (ifile, total commander + t-pod addon) to get access to your iPhone/iPod.
Slanted Lock of course, Typophone Weather, and the Typophone App from the Appstore.
Typophone from Appstore you need for changing the background picture, Typophone Weather is for being able to change the picture in the AppStore App and it's the destination where the LockBackground.jpg will be.
You go into your SlantedLock directory
Code:
private/var/stash/Themes.XXXXXX/SlantedLock.theme/
there you open the Private folder and then the css folder.
Open and Edit the SlantedLock.css
Search for the following code
Code:
background: url("/private/var/mobile/Library/SpringBoard/LockBackground.jpg")
and replace the the part in-between the brackets with the following url:
Code:
"../../../Typophone Weather.theme/LockBackground.jpg"
it should look like this now
Code:
background: url("../../../Typophone Weather.theme/LockBackground.jpg")
It will go three directories backwards and then locate the background in the Typophone Weather folder.
Then you go into the AppStore App of Typophone and change the Wallpaper, respring, et voilą, there's your background 
After that changing your wallpaper is as easy as ABC
It's the best and only solution I know (after around 5 hours of research)
I hope you'll enjoy it
If you like it, it would be great if you "Thanks" me
[NOTE] The other options in Typophone will not affect SlantedLock! [NOTE]

Deactivate the wallpaper, reactivate it to set another one.

