I am not quite sure how use HD images for sprites in cocos2d. I looked through the documentation, and downloaded the newest beta which supports points instead of pixels, but I
I think the "-hd" is for Cocos2D-iPhone, the "@2x" is for icons used by Apple (e.g. Default.png).
If you want to see the difference, 1) setup slightly diff images (e.g diff colour, add minor changes) between normal and the "-hd" version, then 2) run it in simulator with "iPhone" (for iPhone 3 GS) and then with "iPhone(Retina)" (for iPhone 4+) and you should see different one been displayed.
Hope this helps.