I developed an application that uses lots of images on Android.
The app runs once, fills the information on the screen (Layouts, Listviews,
I've ran into this exact problem. The heap is pretty small so these images can get out of control rather quickly in regards to memory. One way is to give the garbage collector a hint to collect memory on a bitmap by calling its recycle method.
Also, the onDestroy method is not guaranteed to get called. You may want to move this logic/clean up into the onPause activity. Check out the Activity Lifecycle diagram/table on this page for more info.