How can i change my background of 4d1?

Depends on what background you want to change ?

m2demo/ui_mp/bg.inc

background image file menu_sp_image

#ifdef DO_THE_CHICKEN_WING
	itemDef
	{
		rect -107 0 854 480
		style 3
		border 0
		ownerdraw 0
		ownerdrawFlag 0
		borderSize 0
		foreColor 1 1 1 1
		backColor 0 0 0 0
		borderColor 0 0 0 0
		outlineColor 0 0 0 0
		origin 0 0
		background mw2_main_sp_image
		decoration 
		type 0
		visible 1
	}
#endif

m2demo/images/ => main image source

Find image you want to make bg (background)

Download Photoshop (Adobe)

Download Nvidia Photoshop Plugins -> DDS Plugin

Copy image into photoshop then save as DDS file -> Use DXT1 no Mipmaps. (if dont work, try res 1280x700 or ARGB8 1280x720 Generate mip maps)

Download some DDS2IWI converter

Find MW2's IWI hex code (some code must be in converted files more likely version or password some kind)

Convert your file and put it in m2demo/images folder as named menu_sp_image.iwi