Ok, I seen someone ask this a little bit earlier, so I tried it myself, and it worked.
Ok first of all, you need these tools needed to mod. I can't post an outside link, so PM me if you need them.
It comes with all the tools needed to edit.
Now that you got those, find the image that you want for your backround.

Now you need to make sure that it is sized to 1280x700.
Once it is resized, find your menu_mp_image file inside IW_03, and extract it to your desktop.
Now you need to convert your iwi file to dds format with iwixdds
Good. Now you can open it in Photoshop, but make sure you installed the plugin included in the ********** file.
When your done, and it looks as gay as mine does, look on the channels tab.

Make sure it is a D3D/DDS Document

Since it has an Alpha channel, save it as ***5 ARGB 8bpp Interpolated Aplha

If it doesn't have an Alpha channel, save it as ***1 RGB 4bpp No Alpha
Now that your done with all the editing, now you need to convert it back to iwi format with dds2iwi bt dragging the file into the app.
Now you need to open a few things to make it work without Hex Editing the file. Copy the directory where the old file is located, and paste it into the
Original file on the IWIFixer

Now do the same thing for the new Files. Then click start
Ok, almost done now. Now repeat the last step, using the CRC Fixer

When you are done, click execute.
Now all of your file is fixed, and ready to be put back into your IW_03 file.
Now it will show up on your screen
Good job, you now have a gay custom background like mine

You can also edit the menu_mp_backround file and do the same thing.
Thank me if I helped.
EDIT: Fail!!!! Wrong section lmao! Plz Move Ty
Move me!
/