You should put 3 smaller pieces with large white mats and antiqued silver frames over the shelf horizontally. Perhaps a antique shell prints (like these) http://www.finerareprints.com/print_detail.html?stock_no=11900
Then anchor the space with dark urns on the floor on either side of the fireplace with large green trees.

I'd go for some object.

I'd get rid of the candles, and find some tall, thin vases of varying heights, that pick up the colors in the artwork.

Or perhaps a wall hanging that is more sculptural, not a painting. Since you've already got some flat artwork on the walls.

Move the shelf down and place a shiny or sculpural oval mirror above it flanked by two sconces

a velvet painting of elvis that cries.

I would put a large vase or other object from Jonathan Adler -- maybe the large tamarind vase.

An oval mirror

ditto on the mirror

3rd on the mirror (maybe an oval or round one)

a design of your own making from wondefulgraffiti.com?

Sunburst mirror?

oh no, not a sunburst. Definitely something sculptural, picking up on the square tile.

A tightly-spaced grid of mirror tiles surface-mounted to black boxes or thick MDF painted black. With mirror tiles form a home improvement store, could even be DIY.

Gives you the sparkle of mirror, but breaks it up and echoes the tile surround beneath it.

But I also very much like the suggestion to hang the dining room painting above the fireplace, and put the rectangular mirror in the dining room.

That shelf is anemic. Put one up that's more robust, thicker and wider, and mount it a few inches lower than what's up there now.

Above that I, too, immediately thought of a starburst mirror. Something sculptural and jewelry-like.

Like this: www.seenonstyle.com/detail.php?p=21925&v=sosdmnsbc&SESSID=a7a3f560e0210c89e226accad399a339

Or this: http://www.kohlerinteriors.com/images/product/large/ccc04953.jpg

See Casamidy mirrors. http://www.casamidy.com/mirrors.html

I agree with bruised about lowering the shelf. Visually, the brick at the top of the fireplace is about at the same level as your table, then comes the base of your painting, then the shelf. If you want to keep the shelf lower it a bit and put something tall on it, like vases to about the same height as your painting.

Or lose it entirely. I'm not sure you actually would need anything on the wall without it. Blank walls are okay, you don't have to put something on every wall. But the shelf does draw your eye to the blank spot above it and it looks like something is missing.

If I were you, I think I would shift the painting on the left. But I would not hang it over the fireplace. I would choose another painting right for the room. Perhap those reproduction of original artwork by Monet Original artwork are your good choice. I found a painting with a good price here: http://dealstudio.com/searchdeals.php?deal_id=68339
Maybe you can try this one.

I'd hang some sort of colorful textile or tapestry for contrast in texture as well as color.

Thanks for all these great ideas. I do think I'm going to remove the shelf. I wanted a real mantle to fit in with the pre-war interior of the condo, but it's too pricy. And for some reason it never occured to me to put the painting there. I'll have to play around with these options.

thanks again - I've been staring at that blank space for a year!