Traffic does take a dip at night - at least mine does. Nothing drastic but it is kind of what you would expect.... it seems to peak some time in the late afternoon thru around 9 or 10 P.M. and then a slow down turn until the next morning when it slowly builds again.
These examples are my local time and the variations are not that drastic. If you see a peek of maybe 3k an hour, your slowest hour should be around 1.5k
As for your layout and your returns... my best advice would be to change your background color and your font color. This alone will not do it but it may make your site feel a little more welcome. Those colors you are running are .... well.... ugly
The other advice I can offer is to try and get your gallery list closer to the top of your page... between the banner and the large ad table, and the other misc. stuff... it's a whole lotta scrolling to find the galleries. The way to keep the surfer and get them clicking is to show them right up front what you have to offer.
Hope this helps ya a little !
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Make $$$ the Diabolical Way