I tried to change via dragging from the Backgrounds and Emblems window, but the icon just goes back to that window rather than changing the folder background.However, I can change the task bar by this drag-n-drop.

Probably it is something about changing ownership permission? if so how to change that?

In /home/mashruf/.gconf/apps/nautilus/preferences/%gconf.xml file it says:, Should I change this file? how?

<?xml version="1.0"?>
<gconf>
    <entry name="click_policy" mtime="1297597800" type="string">
        <stringvalue>single</stringvalue>
    </entry>
    <entry name="default_folder_viewer" mtime="1297597336" type="string">
        <stringvalue>list_view</stringvalue>
    </entry>
    <entry name="media_autorun_x_content_open_folder" mtime="1297534321" type="list" ltype="string">
    </entry>
    <entry name="media_autorun_x_content_ignore" mtime="1297534321" type="list" ltype="string">
    </entry>
    <entry name="media_autorun_x_content_start_app" mtime="1297534321" type="list" ltype="string">
        <li type="string">
            <stringvalue>x-content/software</stringvalue>
        </li>
    </entry>
    <entry name="start_with_location_bar" mtime="1297300028" type="bool" value="true"/>
    <entry name="side_pane_view" mtime="1297269334" type="string">
        <stringvalue>NautilusTreeSidebar</stringvalue>
    </entry>
    <entry name="navigation_window_saved_maximized" mtime="1297600306" type="bool" value="false"/>
    <entry name="navigation_window_saved_geometry" mtime="1297600306" type="string">
        <stringvalue>964x608+59+2</stringvalue>
    </entry>
    <entry name="sidebar_width" mtime="1297390418" type="int" value="192"/>
</gconf>