Convert JPG to BMP for Wallpaper

This issue seems to have been addressed before in numerous round-about ways, but nothing for this specific problem. Here's what I need to do:

I have a folder of 100 JPG files that I need to randomly set as the Windows XP's desktop wallpaper. The change is done through an automated batch file I created - and it's to be used on several systems. I have the code for the random choosing of the JPG - it's the implementation of the wallpaper that I'm having trouble with. Through a "reg" command in the batch file I can use this registry key:
[HKEY_CURRENT_USER\Control Panel\Desktop]  "Wallpaper"
for telling Windows to use the randomly-chosen picture file. Works great, but it only accepts BMP files, and of course my files are all JPGs. In fact, when the registry setting is adjusted through use of Control Panel->Display->Desktop, Windows XP itself converts the JPG to a BMP before adding it to this registry setting. I need to mimic what Windows does, and implement it through use of a batch file, or a set of commands/programs called from the batch file. It has to be automated and work in any XP system I use it.

(1) Either I need an command-line method to convert my JPG to BMP like Windows does
-or-
(2) I need another method for displaying my JPGs as wallpaper directly

My project will go on a CD, be poped it in to a computer, and it will change the wallpaper with near-total automation (besides starting the batch file).

The Restrictions:
I don't want to convert all my JPGs to BMPs (that would be a waste of space).
I don't want to enable Active Desktop as has been suggested elsewhere (waste of resources in my opinion).
I can't install any programs to accomplish this. The environment needs to be untouched. (vbscript, batch, or stand-alone programs ok).
I prefer free programs. :-)

Help appreciated.
LVL 3
bsautter33Asked:
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DanRollinsConnect With a Mentor Commented:
Here's a command-line conversion tool
   ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/jpeg6b32.zip
found on this google groups page:

    http://groups.google.com/group/comp.archives.msdos.announce/browse_thread/thread/295953ef7c81a0f4/50a1cd14eac2f363

Unzip djpeg.exe (it requires no external DLLs other than standard Windows stuff) and use a command line like

    djpeg -bmp somefile.jpg

To output directly to a target directory, use a syntax like:
     djpeg -outfile "c:\some dir\somefile.bmp" somefile.jpg

-- Dan
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Ryan ChongCommented:
Pardon me isn't that Windows allow other picture format like JPEG to be as wallpaper? I thought JPEG image size generally is smaller than a BMP image?
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DanRollinsCommented:
Interesting... apparently the Control Panel copies the JPG file into the user-specific directory and converts it to a BMP file (named Wallpaper1.bmp)

I'll look into a simple batch-file-oriented way to do the conversion, but first tell me:
After replacing the file, how do you get Windows to refresh?  When I change the file, the old wallpaper stays on screen.

-- Dan
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bsautter33Author Commented:
Bravo! That's exactly what I'm looking for.

Allow me to answer your question in return. Updating the wallpaper is done by a relatively simple command:
 
  %windir%\System32\RUNDLL32.EXE user32.dll,UpdatePerUserSystemParameters

Basically, it refreshes the user's settings (wallpaper in this case) without having to logout/in. I tried it on my laptop a few mins ago, and it worked without a hitch. A gem of a command that I had to pull out of a vbscript I had found on the web. My particular situation didn't have need of it, since my batch file reboots at the conclusion of processing, but it'll come in quite handy for other things.
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DanRollinsCommented:
Thanks for the points and the grade... and thanks for the tip about UpdatePerUserSystemParameters :-)
-- Dan
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