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Just tested with getInfoLabel('System.ScreenWidth') and it DOESN'T center the image correctly, while with my code it does... ?? 

 
Ok, so here is an example of estuary's skin addon.xml:

Code:

    <extension point="xbmc.gui.skin" debugging="false">
        <res width="1920" height="1440" aspect="4:3" default="false" folder="xml" />
        <res width="1920" height="1280" aspect="3:2" default="false" folder="xml" />
        <res width="1920" height="1200" aspect="16:10" default="false" folder="xml" />
        <res width="1920" height="1080" aspect="16:9" default="true" folder="xml" />
        <res width="2560" height="1080" aspect="21:9" default="false" folder="xml" />
    </extension>

Imho, simply using the first res doesn't apply to all setups, unless I am wrong and special://skin has a runtime-modified addon xml with simply only the current width/height used.

Just tested with 1 image and it displays only 1 image. So the solution is to set "change banners every" to any value (probably best to highest), and "pause between banners" to 0. 

I 'll probably use this solution.

You can achieve this easily by making a transparent PNG/GIF and moving the image to the far left or right. 

I 've never thought about it, very good idea. I hope its not being drawed of the playback controls and users can't use them.