required resolution for images in android
Android: Background Image Size (in Pixel) which Support All Devices
xxxhdpi: 1280x1920 px
xxhdpi: 960x1600 px
xhdpi: 640x960 px
hdpi: 480x800 px
mdpi: 320x480 px
ldpi: 240x320 px
For understanding multiple supporting screens you have to read http://developer.android.com/guide/practices/screens_support.html
Android Devices Matrices:
ldpi mdpi hdpi xhdpi xxhdpi xxxhdpi
Launcher And Home 36*36 48*48 72*72 96*96 144*144 192*192
Toolbar And Tab 24*24 32*32 48*48 64*64 96*96 128*128
Notification 18*18 24*24 36*36 48*48 72*72 96*96
Background 240*320 320*480 480*800 768*1280 1080 *1920 1440*2560