I know it doesn't because I joined #Help.mIRC, and the background was created fine.
then, on another server, I joined #help.mIRC, and the uppercase H was behind the lowercase one, despite the /background -x command.
Perhaps use this coding to enable network specific BMP.
drawsave %win $+($chan,$network $+ .bmp)
background $chan $+($chan,$network $+ .bmp)
.remove $+($chan,$network $+ .bmp)