Develop Reference

What is the full screen image sizes for an Android app? (Android Studio)

I have a background image in an activity, its size was set to 1280x920, as adviced in Background image crashes android. This works perfectly.
I also have a full screen activity (sticky immersive), with the same configuration, the image is "stretched" out vertically. What should be the size of the image so that there is no scaling or stretching at all? (While the image occupies the full screen, off course)

It depends on screen. Android supports multiple screens. That's way in project we have x-hdpi, hdpi, xx-hdpi folder. As source:
https://developer.android.com/guide/practices/screens_support.html
https://developer.android.com/training/multiscreen/screensizes.html

There is no one size. Every Android device has different screen sizes. What you need to do instead is provide multiple sizes and accept some stretching or shrinking on odd sized devices. See https://developer.android.com/guide/practices/screens_support.html for more advice.

Pixel size of images

i want a default pixel size of image in this size should be adaptable for all type of screen size.Anyone can help me please, How to fix image in all type of mobile screen. it cant be stretched out.
thanks in advance.

There is no default pixel size.
You will either have to design images for each bucket you wish to support, or try and create a 9-patch for your images.
The Supporting Multiple Screens document has everything you need to having your UI scale nicely across devices.

Sorry, You cannot have a Default size...
If you are developing a Mobile app, then you should detect the User's screen size and use an appropriate dimension of the picture there....
Have at least three different variations of the same picture for different screen sizes..
Android Has an inbuilt way of doing this Have a look at this : Supporting Different Screen Sizes

Design of the home screen in android

I am new to android and having requirement to design my home screen for the app.which look like as shown below.
Home screen image.
as the picture shown above contain five button which having five different drawable image with different design and different size.the same layout while running in one device look different compare to the one running on other device.different device means here the different screen size and different resolution.so now the question is how to design that size and resolution of image and layout so its fit with any screen size and resolution on any device android device available in market and never expand or collapse with different screen size.i am testing the app in three different screen size device and resolution also and the look change for every device.Any kind of reply will be very helpful.

Use RelativeLayouts to separate the different boxes.
Apart from that, whatever you asked is broad.You should refer the Designing for Multiple Screens documentation available at android.

You should follow the advice that's detailed in the documentation on how to design for different screen sizes. You're going to have to have more than one image to handle all your scenarios obviously.
http://developer.android.com/guide/practices/screens_support.html

place Button background images in 4 resolutions in the res folder as ldpe mdpi hdpi and xhpdi .use Dip insted of dp.

Trouble understanding how Android screen sizes/density works

I'm new to Android development and I'm having some trouble deciding what resolution to tell the designer to send me the images.
I'm supposed to be focused on devices with medium screen size and hdpi density. Specifically for the Android Galaxy S II, which has a 480x800 screen.
Could anyone point me in the right direction.

First you should skim this entire article:
http://developer.android.com/guide/practices/screens_support.html
Then tell your designer to make their designs on a 480x800 palette. So for example if they are going to design a screen for you that has a button that is half the width of the screen, they should create an image that is 240px wide. You should then put that image in your HDPI folder.
I have my designer create 'comps' or designs for an entire screen that are 480x800px, and then deliver all the assets as the exact size they are in the image.

Android screen resoulution problem

I'm writing a game for Android and can now test it on a second device, the Nexus 1. The game uses fix pixel-values, just using bigger cutouts of the background for high-res devices. So I thought there would be no problems. Somehow, however, the nexus 1 is making a specific image bigger than it should be (261*66 instead of 174*44). The picture itself as a resource is 174*44, so it's being stretched. Why? What can I do against it?
Edit:
Spritesheet = BitmapFactory.decodeResource(res,
R.drawable.bird_spritesheet);
Is the used code.
Edit 2:
Is there no way to tell the software to just use the size the picture is? I don't want to bloat my software by adding multiple pictures (/drawable-hdpi/ answer).
The pictures are supposed to be smaller on bigger screens.

what drawables folder to you have the picture in? If you put a copy of it in drawables-hdpi I think it will show up real size. It is really better to set things up in such as way that the final size in pixels it ends up is unimportant. Using pixels values is going to ensure that your app looks wrong on at least some of the screen sizes out there.

Because Android runs on multiple screen sizes and you use device independent pixels (DIPs), images get scaled to ensure they look the same on all devices. To avoid this, you can provide alternative resources for high density screens (in your case) and for low density screens.
More info about screens here