Yes, it is not problem to make screenshots yourself. But every utility for making thumbnail preview of web pages is *SLOW*. Even if you build dedicated server, it take whole month to generate thumbnails for small catalogue with 1000 links. And you spend nearly same amount of money on electricity as on subscription on Thumbnail.cz. If you will count price of hardware and your time, you get much higher cost.
And if you want to make faster system, you must spend a lot of time (and money) in development. And the results are still unsure.