How to reduce the memory usage of background images?
<p>In my app, I put PNG images into drawable directory and use them as activity backgrounds. Since the background image usually cover the whole screen, they are big and use a lot of memory. 
My background images are total opaque. Is there any way that I can optimize those background images, so they can take less memory? I heard that we can reduce the color-depth of the images so they take less memory when loaded, but I don't know how to reduce the color-depth? What tool can I use?</p>

<p>Thanks.</p>
