Yes, I did enable External FIRM in Luma3ds settings. The cxi is named TwlBg.cxi and located in folder luma/sysmodules/ but the result is like the top screen being cropped.
I also tried to rename the cxi to Widescreen.cxi and moved it to _nds/TWiLightMenu/twlBg/ but the result is I lost...
Sorry for the late reply. I've done following your instructions above. But the widescreen options in Twl settings were removed. Did I do something wrong? Anyway thank you for the reply