Android, how to set imageview with percentage in relativelayout

I want to build a UI like this http://postimg.cc/image/fihidv1rv/ . Here is my xml code for it, for my design, I want "EFFECT" & "CAMERA" to combine as one ImageView like "SHOP" in the link so there will be total 5 ImageViews, and I set the id as they named in the link

the problem is, how can I set the height and width with percentage?

effect+camrea: height 25%,width 100%

collage: height 25%,width 50%

draw: height 25%,width 50%

photo: height 50%,width 50%

shop: height 25%,width 100%

<RelativeLayout android:id="@+id/mainContent" android:layout_width="fill_parent" android:layout_height="fill_parent"
    android:orientation="horizontal" android:background="#ffffff">
    <ImageView 
        android:id="@+id/img_effect+camera" 
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true" 
        android:src="@drawable/a" />
    <ImageView 
        android:id="@+id/img_collage" 
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true" 
        android:layout_below="@+id/img_effect+camera"
        android:src="@drawable/b" />
    <ImageView 
        android:id="@+id/img_draw" 
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_below="@+id/img_effect+camera" 
        android:layout_toRightOf="@+id/img_collage"
        android:src="@drawable/c" />
    <ImageView 
        android:id="@+id/img_photo" 
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_toRightOf="@+id/img_collage"