Well I dont have austerity on my pc anymore but for what I can recall:
Austerity has an .ini file that will set its wallpaper automaticly as you load the theme.
Look in the austerity directory for the wallpaper(correct resolution) get the name, then find(or make) your new wallpaper and rename it to the austerity name, and replace the old austerity wallpaper for the one you just renamed.(thats that)
next time you start austerity your new wallpaper should show.

or

You could change the wallpaper name on the .ini file to a new one (just make sure you put the new wallpaper on the same directory)
either way should work.