The collage is too dark, first I thought something was wrong because the image was black, but then I had to move my screen to see the boxes. Also depending what this is really for (a website?) it might be too small - show your visitors your stuff, don't hide it behind too small thumbnails.
Also, a good idea is to apply a bottom drop shadow that is white with an opacity that is good (like 10-20% perhaps?) and the inner shadows should have an offset of 90° and slightly smaller size and distance.
The last thing is that you should try a lighter background - if it's not that dark on your screen, you have to set your gamma/contrast down.