We read from left to right. It's so ingrained you can't really help starting at the top left to process a piece of text. However, we like looking at pictures more. Select boxes in a form do the same thing (perhaps not because we like em more, but rather they're more complex than other form elements): we're drawn to look at them.
Your second example really drags my eye to the right edge of the screen. The very light contrast of the text next to the images only reinforces that. Our eyes really like to move to the right to continue looking at something. However, in your second example there is nothing there. So, in stead, our eyes have to skip over the text next to image to start processing that. Moving from left to right, we again end up at a thumbnail and the edge of the screen. So, again, we have to force our self to skip back to process more of the screen in a zig-zag manner.
Your first example works better because our eyes are almost naturally drawn to each thumbnail, keeping our eyes from leaving the screen.
Basically it's one of the laws of composition. You can make eyes leave the area by putting something really interesting on the right, but a good image keeps eyes bouncing around inside it. A similar thing happens for instance when you put a person on the right edge of a photograph and have him looking to the right: it's so natural to try to look at what that person is looking at we almost immediately stop looking at the photograph.