I want to set my background (on a Win XP machine) through the command
prompt (ie mspaint.exe /b c:\temp\mypic.bmp) would set mypic.bmp as my
current background using mspaint.  I also want to do it without any
third party software or code, only using standard windows programs.

Create a file with the following contents:

REGEDIT4

[HKEY_CURRENT_USER\Control Panel\Desktop]
"Wallpaper"="c:\\path\\picture.bmp"

And save it under some name, say wallpaper.reg.

Then execute the following command:

regedit /s wallpaper.reg

The creation of the file can be easily automated with simple commands
like echo "xxx" >> filename.

Hope this helps.

Here is a much safer way to do it. The default location and name for
the background image is c:\Documents and Settings\[username]\Local
Settings\Application Data\Microsoft\Wallpaper1.bmp.

The only that I need to know is what you intend on using it for. Will
it be regularly changing with different filenames? Do you want it to
run automatically?

The bast way to do it is manually, as setting up a script with
variables is a bit of a pain. Type the following:

Copy /Y "[path and name of the image you want to use]" "c:\Documents
and Settings\[username]\Local Settings\Application Data\Microsoft\"
Wallpaper1.bmp

This copies a file (should already be a .bmp) to the directory where
windows stores the desktop image, and overwrites the old one without
requiring a prompt. The name of the file is seperate so that the
original file will be renamed when it arrives at the destination
directory.

If you want an automatic scripts with variables, you need to ask for more than $2.

There is no Microsoft-supplied program to do this from the command
line.  And unfortunately neither of the answers above will work
either.  They change the the entry in the Registry but the background
will not change until the next time you log in.

The only way to do this reliably is with a program that calls
SystemParametersInfo with function code SPI_SETDESKWALLPAPER and a
string with the pathname of the file you wish to use as the new
background.  I have written a program that sets the desktop background
to the filename given on the command line (if that fails for some
reason, the old background is put back).  It uses only documented
interfaces and makes the call in such a way that the registry is not
changed.

Here is the code.  I had to declare SPI_GETDESKWALLPAPER directly
because for some reason my version of the SDK doesn't have it defined,
even though it's documented!

#include <windows.h>
#include <stdio.h>

const SPI_GETDESKWALLPAPER=115;

void printusage(char *program)

{

	fprintf(stderr, "Usage:  %s background-file.bmp\n", program);
	fprintf(stderr, "   Changes desktop background to background-file\n");
	return;

}

int main(int argc, char *argp[])

{

	DWORD dResult;
	BOOL result;
	char oldWallPaper[255];

	if (argc != 2) {
		printusage(argp[0]);
		return 1;
	}

	result = SystemParametersInfo(
		SPI_GETDESKWALLPAPER,
		sizeof(oldWallPaper)-1,
		oldWallPaper,
		0);

	fprintf(stderr, "Current desktop background is %s\n", oldWallPaper);

	result = SystemParametersInfo(
		SPI_SETDESKWALLPAPER,
		0,
		argp[1],
		0);

	if (!result) {
		dResult = GetLastError();
		fprintf(stderr, "Attempt to set new desktop background failed; code
%d\n", dResult);
		fprintf(stderr, "Will restore prior setting (%s)\n", oldWallPaper);

		result = SystemParametersInfo(
			SPI_SETDESKWALLPAPER,
			0,
			oldWallPaper,
			0);

		return 2;
	}

	fprintf(stderr, "Desktop background changed to %s\n", argp[1]);
	return 0;

}