Cached files are indeed stored in /data/data/my_app_package/cache
Make sure to store the files using the following method:
String cacheDir = context.getCacheDir();
File imageFile = new File(cacheDir, "image1.jpg");
FileOutputStream out = new FileOutputStream(imageFile);
out.write(imagebuffer, 0, imagebufferlength);
where imagebuffer[] contains image data in byte format and imagebufferlength is the length of the content to be written to the FileOutputStream.
Now, you may look at DDMS File Explorer or do an "adb shell" and cd to /data/data/my_app_package/cache and do an "ls". You will find the image files you have stored through code in this directory.
Moreover, from Android documentation:
If you'd like to cache some data, rather than store it persistently, you should use getCacheDir() to open a File that represents the internal directory where your application should save temporary cache files.
When the device is low on internal storage space, Android may delete these cache files to recover space. However, you should not rely on the system to clean up these files for you. You should always maintain the cache files yourself and stay within a reasonable limit of space consumed, such as 1MB. When the user uninstalls your application, these files are removed.