LruBitmapCache.java

package your.awesome.app;

import android.graphics.Bitmap;
import android.support.v4.util.LruCache;
import com.android.volley.toolbox.ImageLoader.ImageCache;

public class LruBitmapCache extends LruCache<String, Bitmap> implements ImageCache {

	public LruBitmapCache(int maxSize) {
		super(maxSize);
	}
	
	@Override
	protected int sizeOf(String key, Bitmap value) {
		return value.getRowBytes() * value.getHeight();
	}
	
	@Override
	public Bitmap getBitmap(String url) {
		return get(url);
	}

	@Override
	public void putBitmap(String url, Bitmap bitmap) {
		put(url, bitmap);
	}

}

@ArnaudCourbiere

You can take a look here at the code of the Google way of do this:
http://developer.android.com/training/volley/request.html

After all I think it depends on how you use volley. It makes sense on apps like Instagram or Google Play Store where you have the screen full of images. If you use Volley for request images but you have 1 or 2 images in each activity and they are not really loaded heavily... I have no idea how to work on this cache image stuff. Probably a few MB is enough.