Give your layout for the views created by the ListAdapter a fixed width and height sufficient for the largest dimensions, then position the ImageView centered inside e.g.:
<RelativeLayout android:id="@+id/RelativeLayout01" android:layout_width="100dip"
android:layout_height="80dip" xmlns:android="http://schemas.android.com/apk/res/android">
<ImageView android:id="@+id/imageView" android:layout_width="fill_parent" android:layout_height="fill_parent"
android:scaleType="centerInside"
>
</ImageView>
</RelativeLayout>