Maybe you should use some of the pixels yourself so it doesn't look so empty. Use it to demonstrate how it works, make your ads link to a page which explains the concept.
Also, the pixel map is too big, I think you want all the million pixels to be above the fold, meaning you don't want to scroll to see the lower pixels. Make the squares smaller for display and large for selecting to purchase.
It is a "standard" 100x100 size, at the moment it is very profitable to buy pixels when the cost is 0.000075BTC per pixel so I will leave the customers to do as they please, "faking" bought pixels would be unfair and give the wrong impression to potential buyers and I want to build the website on mutual trust, I want every customer to be certain that I got his back and I am not going to swindle people out of their money.