ImageButton showing artifacts in transparent image

Question

Hello,

I have a layout like this:

<LinearLayout android:background="mybackgroundimage.jpg">
    ...
    <LinearLayout android:weight=1> <!-- another layout to group the 
               images below and give it weight=1 -->
        <ImageButton android:background="item1bg.png" .../>
        <ImageButton android:background="item2bg.png" .../>
    </LinearLayout>
</LinearLayout>

The images item1bg and item2bg fade from full image to completely transparent. The desired effect is to have the image blending to the background.

I've verified the images and they look OK when you open them with whatever. But when applied in the button, they show artifacts in the blending section: some green vertical transparent lines.

I tried setAlpha(0) but the artifacts remain.

Any ideas on why this happens and/or how to solve it?

Thanks! Llappall