The ldpi, mdpi and hdpi refer to screen density, which means how much pixels can fit into a single inch.
the ratio in pixels between them is:
- ldpi = 1:0.75
- mdpi = 1:1
- hdpi = 1:1.5
- xhdpi = 1:2
- xxhdpi = 1:3
- xxxhdpi = 1:4
so lets take an image with about the size of 100X100:
- for mdpi it should be 100X100
- for ldpi it should be 75X75
- for hdpi it should be 150X150
- for xhdpi it should be 200X200
- for xxhdpi it should be 300X300
- for xxxhdpi it should be 400X400
this way, for screens with the same size but different DPI, all the images seem the same size on screen.
Also you have multiple screen size types small, normal, large, xlarge and each one of them can be ldpi, mdpi, hdpi, xhdpi, xxhdpi (Nexus 10) or xxxhdpi.
You can try to create a splash screen image that fit to each and every screen type
which gives you 4*5 = 20 different images (it seems to much for me).
For now only the Nexus 10 is at the xxhdpi category.