Q:

image size (drawable-hdpi/ldpi/mdpi/xhdpi)

Study android, now I've some images to put into drawable dirs (hdpi/ldpi/mdpi/xhdpi). I read a lot but again I don't understand. Which size should be an image for every directories?

A:

Hope this will help...
mdpi is the reference density -- that is, 1 px on an mdpi display is equal to 1 dip. The ratio for asset scaling is:
ldpi | mdpi | hdpi | xhdpi | xxhdpi | xxxhdpi
0.75 | 1    | 1.5  | 2     | 3      | 4

Although you don't really need to worry about tvdpi unless you're developing specifically for Google TV or the original Nexus 7 -- but even Google recommends simply using hdpi assets. You probably don't need to worry about xxhdpi either (although it never hurts, and at least the launcher icon should be provided at xxhdpi), and xxxhdpi is just a constant in the source code right now (no devices use it, nor do I expect any to for a while, if ever), so it's safe to ignore as well.
What this means is if you're doing a 48dip image and plan to support up to xhdpi resolution, you should start with a 96px image (144px if you want native assets for xxhdpi) and make the following images for the densities:
ldpi    | mdpi    | hdpi    | xhdpi     | xxhdpi    | xxxhdpi
36 x 36 | 48 x 48 | 72 x 72 | 96 x 96   | 144 x 144 | 192 x 192

And these should display at roughly the same size on any device, provided you've placed these in density-specific folders (e.g. drawable-xhdpi, drawable-hdpi, etc.)
For reference, the pixel densities for these are:
ldpi  | mdpi  | hdpi  | xhdpi  | xxhdpi  | xxxhdpi
120   | 160   | 240   | 320    | 480     | 640

A:

MDPI - 32px
HDPI - 48px
XHDPI- 64px

This Cheat Sheet might be handy for you. check the image :-)

A:

I have a full explanation already posted here 
Basically, General guidelines for designing images are:
ldpi is 0.75x dimensions of mdpi
hdpi is 1.5x dimensions of mdpi
xhdpi is 2x dimensinons of mdpi

Usually, I design mdpi images for a 320x480 screen and then multiply the dimensions as per the above rules to get images for other resolutions.
Please refer to the full explanation for a more detailed answer.

