The following webpage has all you need to know to change that picture:
http://www.infocellar.com/winxp/customize-system-properties.htm
I'll briefly summarize the important steps here, assuming you're using Windows XP.
1) Have an .bmp (Bitmap Image) file of size 180x120. Rename it "OEMlogo.bmp". Make sure the complete name of the file is "OEMlogo.bmp", not "OEMlogo.bmp.bmp" or others.
2) Create a file named "OEMinfo.ini", using notepad or any text editor. Copy and paste the following 7 lines into that file:
[General]
Manufacturer=PC Doc
Model=Dell XPS 450R (1.4 MHz)
[Support Information]
Line1=John Doe
Line2=200 Main St.
Line3=Sterling, MN 20987
(You can modify the information on right hand side of the "=" to anything you like, but you have to keep the words on the left hand side.)
3) Put both files into the directory C:\Windows\System32.
4) Now check "System Properties" again. You should see your custom logo.