In Tiger you could select background images in Preferences, but that went away in Leopard. The .terminal files can be exported and imported as a xml property list. The plist "key" node value for the background image is BackgroundImagePath followed by a "data" node that contains a base64 encoded binary property list that has a "string" node pointing to an image file.
<key>BackgroundImagePath</key>
<data>
...base64 encoded binary plist here....
</data>
You can use openssl to decode the data content.
openssl enc -d -base64
Then convert the binary plist to xml using plutil.
plutil -convert xml1
You will see something like this...
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>$archiver</key>
<string>NSKeyedArchiver</string>
<key>$objects</key>
<array>
<string>$null</string>
<string>/Users/user/Pictures/myimage.png</string>
</array>
<key>$top</key>
<dict>
<key>root</key>
<dict>
<key>CF$UID</key>
<integer>1</integer>
</dict>
</dict>
<key>$version</key>
<integer>100000</integer>
</dict>
</plist>
Change the image path and convert the xml plist back to binary and base64 encode it.
plutil -convert binary1
openssl enc -base64
Then set the value of the "data" node to the base64 string.