Ok this question has been asked way too many times both in the forums AND on IRC, so im gonna write a tutorial:
Ok to start, you need
WinXIP not WinZIP or WinRAR, this is the program used to edit the .xip files that UIX and the m$ dash uses.
Start by FTP'ing over your \user.interface.x.dash\xips\default.xip over from your xbox.
Next run the program winXIP, then go File -> Open
Open up defaut.xip
You will get a bunch of config.xap, default.xap, dvd.xap, etc...
The 1 we are looking for is default.xap
Click on it and go edit -> extract
Extract it to the desktop then open default.xap with notepad, yes notepad...
Now you will see a bunch of code. Dont touch any of it, unless i tell you to.
Ok now go Ctrl + F or find. Type this in:
CODE
DEF theBackground Background
Hit enter and you will see somthing like this:
CODE
DEF theBackground Background
{
skyColor 0 0 0
//backdrop ImageTexture { url "xboxlogo.xbx" }
}
Now change that so it looks like this:
CODE
DEF theBackground Background
{
skyColor 0 0 0
backdrop ImageTexture { url "background.xbx" }
}
Save, and open up default.xip again, go edit -> insert.
When it asks for an overwrite, go YES.
Then save, and ftp it back to the \user.interface.x.dash\xips\
It will ask for an overwrite, check the file size, it should be just smaller than the original
Now reboot UIX and your done