For each image you need to create multiple images to support different screen resolution (mdpi, hdpi, xhdpi, xxhdpi). Then you can place them in appropriate folders you have created. In drawable folder you keep the xmls which internally uses other drawable folders.
The link below will give you idea about supporting different screens:
http://developer.android.com/guide/practices/screens_support.html
The below link will give clear pictures about icons:
http://developer.android.com/design/style/iconography.html
You can use Android Asset studio to generate icons. Follow link below:
http://android-ui-utils.googlecode.com/hg/asset-studio/dist/index.html
In case you cannot get different images for different screen resolutions, just put them in drawable folder and it will be picked for any device. But the look and feel of your application wont be good on different devices.
Hope it helps you.