This is the way i found to change backgrounds of 5 to 9 desktops

Quick Steps:
1)Download WinSCP & Install It (in your PC)
http://winscp.net/eng/index.php


2)Download OpenSSH Server & Install It (in your N900)
you can find it in application manager on ur n900

type easy password when it ask password in the installation Example:123456

3)Open X-Terminal, type "root" then Next "ifconfig"
Check your Phone's IP Address (in your N900)

its is next to the line WLAN0: inet addr: 192.168.214.1 (it should be like this) Note down this

4)Open WinSCP & Enter: (In your PC)


Hostname: enter your phone's IP address u get from ifconfig example: "192.168.214.1" without qutes

Port: 22
User name: "root"
Password: The OpenSSH Installation Pass: eg "123456"
Private key: LEAVE IT BLANK
Protocol: SFTP, Tick SCP Fallback
LOGIN....



5.find home/ user / .backgrounds

u can see backgrounds-1.png to backgrounds-4.png

drop ur wallpapers there make sure that they are in .png format

and rename them to background-5.png , background-6.png , background-7.png , background-8.png , background-9.png

exit winscp

now you can see wallpapers on ur desktops



if u want to do it in ur phone install filebox in ur n900 in settings mark show hidden files.u can see .backgrounds folder now.& add ur background there like i said in step no:5