Following crocboy link is correct, and i suggest you to try including more buckets on your app. For example, in my applications i use also this:
drawable-sw600dp-mdpi
drawable-sw720dp-mdpi
In this buckets, put images that match tablets screen, for example in drawable-sw600dp-mdpi insert a background with this size 1280x800 and for drawable-sw720dp-mdpi a background with this size 1920x1200.
It's quite difficult to decide wich tablet to target, because we have different screen size.
EDIT
"application have to support all devices. So, the drawable folder images also have to support for all the devices. How to achieve this?".
Basically Android create this 4 folder for drawable:
drawable-hdpi
drawable-ldpi
drawable-mdpi
drawable-xhdpi
(To be picky, low drawable-ldpi is not quite used, because small device are loosing appeal)But due to density and screen size you can have:
drawable-large-mdpi
drawable-large-hdpi
drawable-large-xhpdi
drawable-xlarge-hdpi
drawable-xlarge-mdpi
And so on with various combination! If you are looking for image perfection (like my graphics collegue, wich is an iOS psychopath) you have to create images for every type of screen, and put it on relative drawable bucket.