1. Create a blank document with width*Height as 2475*1870 keeping background as transparent.
2. Now create your own background like the one I create for myself and save it as jpeg with 220 dpi. I named my file as "Metal.jpg".
3. Now Create a metadata file "Metal.metadata.xml" for Metal.jpg which has the folowing code :
"<?xml version="1.0" encoding="UTF-8"?>
<PSEContent xmlns="http://ns.adobe.com/PSEContent/2.0/">
<name value="$$$/content/backgrounds/Metal=Metal" />
<type value="background" />
<category value="$$$/content/category/Event=Event" /><category value="$$$/content/category/Color=Color">
<subcategory value="$$$/content/category/Color/Blue=Blue" />
</category>
<category value="$$$/content/category/Style=Style">
<subcategory value="$$$/content/category/Style/Teen=Teen" />
<subcategory value="$$$/content/category/Style/General=General" />
<subcategory value="$$$/content/category/Style/Retro=Retro" />
<subcategory value="$$$/content/category/Style/Urban=Urban" />
<subcategory value="$$$/content/category/Style/Colorful=Colorful" />
</category>
<category value="$$$/content/category/Mood=Mood" />
<keyword value="$$$/content/keyword/Stripes=Stripes" />
<keyword value="$$$/content/keyword/Splat=Splat" />
</PSEContent>"
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4. Please note that whatever name you give to your jpeg file say "<filename>.jpg" then your metadata file name should be "<filename>.metadata.xml".
Also note in the xml code, my filename has to be replaced here :
<name value="$$$/content/backgrounds/<filename>=<filename> />
Also note as i wanted my background to appear under category "Color" and sub category "Blue", so i used :
<category value="$$$/content/category/Color=Color">
<subcategory value="$$$/content/category/Color/Blue=Blue" />
So you can customize by replacing the categories and subcategories (existing in Content pane of Editor) as per your needs.
On XP : C:\Documents and Settings\All Users\Application Data\Adobe\Photoshop Elements\8.0\Photo Creations\backgrounds
On Win 7 : C:\ProgramData\Adobe\Photoshop Elements\8.0\Photo Creations\backgrounds
Please make sure that your Editor is closed and you relaunch it to see the background created in your content pane.
If you still do not find it in the content pane in Background category, close Editor and rename the file : "C:\Documents and Settings\All Users\Application Data\Adobe\Photoshop Elements\8.0\Locale\en_US\MediaDatabase.db3" followed by application relaunch. The launch may take sometime as application creates thumbnails for your background at the location where you have created your jpeg.