Here is the code for the displaying an image in the background.
Code:
<control type="largeimage">
<include>Dimensions_Fullscreen</include>
<fadetime>400</fadetime>
<texture background="true">$INFO[Container(9000).ListItem.Thumb]</texture>
<include>Animation_CommonFade</include>
<aspectratio align="bottom">keep</aspectratio>
</control>
<control type="image">
<include>Dimensions_Fullscreen</include>
<texture>filters/home_vignette.png</texture>
</control>
The first control tag (largeimage) is your actual background. The text between the texture tags is what is actually being put in the background.
Container(9000) is the home menu object (not exactly, but in this case yes.). More precisely the id of the control that holds the home menu is 9000 and
Container() simply references that.
ListItem refers to the currently selected item in the home menu and
Thumb refers to the
<thumb> tag of the item.
First thing you will most likely have to do is go into Home.xml and replace an image control that loads a static background. I will look fairly similar to the 'image' control in the above chunk of code.
Then you will need to modify the home menu list (It's an include named HomeIcon if memory serves correctly) to contain a thumb tag with the path to your background image.
Use that string to tell XBMC to look in the current skin's root folder.
So if your background was located in Alaska/media/backdrops/video.jpg, your thumb tag would resemble this (for the video list item)
Code:
<thumb>special://skin/media/backdrops/video.jpg
Also use this wiki article for guidance while doing this.
http://wiki.xbmc.org/index.php?title=Skinning_XBMC
Good luck.