Very beginner question ! but can't figured out...
-> Just want to add some background overlay behind my pop image...
if you had a clue for me.. 
Code
<action name="open">
set(plugin[%1].visible,true);
tween(plugin[%1].alpha,1);
</action>
<action name="close">
tween(plugin[%1].alpha,0,0.5,default, set(plugin[%1].visible,false) );
</action>
<scene>
....
<!-- HOTSPOTS! -->
<!-- 1 -->
<hotspot name="image"
style="skin_hotspotstyle"
ath="-15.519" atv="40.127"
alpha="0.3"
onover="tween(alpha,1);tween(scale,1);"
onout="tween(alpha,1);tween(scale,1);"
onclick="action(open,openimage, overlay);"
/>
<layer name="overlay" type="container"
bgcolor="0x000000"
bgalpha="0.5"
align="center"
bg="true"
width="100%" height="100%"
visible="false"
zorder1="0"
onclick="close(get(name));"
/>
<plugin name="openimage"
url="my_image.png"
width="100%" height="prop"
align="center"
scale="1" alpha="0"
zorder1="1"
onclick="close(get(name));"
/>
</scene>