Question: I have some images that I want to use in my application ( 150 - 200 of them ).
What's the best place to store them? 

* assets folder
* drawable
* why there

Answer: 
if you must put these images into your application pack, choose drawable that images will be compressed.
if you can download when required, store them in sd card is recommend.
You should read the Android article about [Supporting Different Screen Sizes](http://developer.android.com/training/multiscreen/screensizes.html), they show you the benefits of using the Android frame work for that task. If you wish to use assets folder or other solutions like dynamically download the images from your site, it's up to you to write your own implementation to handle all the problems that are describe in the article.