How to create android's application supporting multiple screen size

I look over some guide and tutorial but my problem still exists. I develop this application using small image to create button, text and background and i put it in res/drawable-ldpi directory. Then i see on web that i have to make images with this proportion: 3:4:6. So if my images were small, to create them for medium screen size, i have to take dimension of small image and multiple for 4/3, and for large screen multiple for 6/3=2. I see these here: Providing Resource. Now when i create just some images for normal screen ( medium ) i launch emulator with screen density 160 or 200, but the image still remain that used in ldpi. What can i do? Can you explain me well what is the problem with these multiple screen and how to solve this? This is a part of my android manifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xm开发者_运维知识库lns:android="http://schemas.android.com/apk/res/android"
package="spazio.digitale.com" android:versionCode="1"
android:versionName="1.0" android:installLocation="auto"
android:screenOrientation="portrait"
>
<uses-sdk android:minSdkVersion="7" android:maxSdkVersion="11" />
<uses-permission android:name="android.permission.INTERNET" />
<supports-screens android:normalScreens="true"
    android:smallScreens="true" android:largeScreens="true"
    android:anyDensity="true" />

use to pixels scale to design.. put this in manifest

<supports-screens
android:largeScreens="true"
android:normalScreens="true"
android:smallScreens="true"
android:anyDensity="true"
/>

For button, create images using 9-patch tool.

With bitmaps, the rule of thumb is to start with the highest density first, and then scale it down, because it's always easier to scale down bitmaps than to scale them up.

Also, you seem to be confusing screen density with screen size. Do not worry about the screen size for now (unless you have one background image that takes the entire screen size, which I doubt). And no, the ratio you mentioned has nothing to do with screen sizes, that only applies to screen densities. Fix the density issue first and foremost. Then, once that's fixed, you can focus on making sure the layout fits the size.

And don't even bother with ldpi, that's only 0.2% of the market of handsets. You're just wasting your time optimizing for that density.