Ubuntu 11.10 Wallpaper Changer

Finally I'll post some programming. It turns out that I was searching for my Ubuntu 11.10 to have a wallpaper changer, and I wondered how Ubuntu make the default "automatic wallpaper changes" (look at the image below, also, you'll need to select the picture with the clock to make this work [thanks Labago]):

Well, turns out there's a file in /usr/share/backgrounds/contest called 'background-1.xml', which contains all the wallpapers, the times shown -static time-, and the transition time. Well, I made a python application to modify it at will (it'll create a new file background-1.xml, you only need to replace the current with this one).
The code is (I want to thanks to my friend Esteban Ordano by his help):

You can find it the code above on this repository (I think this will be more updated). Then simply put in the console:
python wallpaper_changer.py "folder" "time_d" "time_t"
Where:
folder: Put right here the folder where your wallpapers are stored (By default, the same folder where you have the file .py).
time_d: Time of stay of a wallpaper in seconds (By default, 1795.0 something like 30 minutes).
time_f: Time of transition in seconds (By default, 5.0 seconds).
If you want everything in default, put simply:
python wallpaper_changer.py
Other example (this one will search in the folder of the path, and will let time_d by default, but will change time_f):

python wallpaper changer.py '/media/sda3/Random\ wallpapers' '' '3'

I have the care to take only files with extensions 'jpg', 'png' and 'bmp' (the basics), but you can change that. And one more thing, you need to understand that every time that you modify the folder with the stored wallpapers, you need to execute again this script.
Well, that's all.
Note: I have the feeling that in Ubuntu 12.04, the name of the file is "precise.xml" instead of "background-1.xml" and in Ubuntu 14.04 it is "trusty.xml". Can you catch the deal?.
Greetings and have a nice wallpaper-change day.
PS: Sorry by the comments on the code in spanish :P.