android memory management in activity lifecycle

My question is a little bit complicated.

I want to understand how the app treats resources (especially images for backgrounds, buttons, etc.) when Activity is started, then suspended.

For example, I start Activity A, it shows all the images on the screen, eats its memory, then another Activity B is started and A is suspended. What happens to all the images, resources, etc.? When they are freed? How can I take control over them? Should I not keep the Activity A in the memory and remove it from the Activity stack?

If you need any clarification for my questions, pls write me!

Thanks in advance! Danail