The thing with Cocos2d-x, if you're developing in XCode, is to add the images directories under Resources as actual directories and not groups. Once that's done, in your AppDelegate.cpp, you can switch based upon screen size to different directories based on resolutions. As per the example:
vector<string> searchPath;
CCSize frameSize = view->getFrameSize();
if (frameSize.height > 640) {
searchPath.push_back("hd);
}
This will effectively ignore asests of the wrong size and only load asets of the correct size.
If you're not developing in XCode, then you need to alter you build file depending upon the resolution the target is aimed at.