Looks at my little chest, looks at Zeldas little chest...
Still puzzled as to what the problem is. I think maybe your art style needs more detail if your upping the resolution to such a degree, you need to tarnish it a bit, try and put some texture onto it. For example add a wood grain to the wood, darken the gold and highlight the edges.
But shading is vital, it's the only way you can make a 2D game appear 3D. You should define a strict system, as I said, 1/3rd dark for the front, 2/3 light for the top, based on if you were drawing a box.
Perhaps it would help if you posted a screenshot with more of your sprites, it is a style problem, so you'll have to draw all your sprites in the same way, but we need to see what your doing wrong - really the only thing wrong with your chest is that the shading is not bold enough IMO.