When many browsers (such as Netscape) render some of the images (such as the First Flight picture), the sky has a funny 'banded' appearance. In the graphics world, this is known as "posterization". This posterization is not a part of my images, but a problem in rendering the image by the browser.
The problem arises because browsers frequently limit their color table to 256 colors. There are only 256 grays in my images, so that shouldn't be a problem. However, several colors are reserved for things like link color, accent colors for the browser icons, and perhaps other colors as well. So, there are fewer than 256 distinct grays left to display my images.
If you want to see a better quality rendering, copy the image to your home drive, then use a program like Photoshop or JPEG-View to look at them. These files will employ a larger color table, if available, to show you a higher quality image. If you are stuck on a system that has only 256 colors in the first place, you may need to shift your monitor to grayscale to get a full compliment of grays.
I work hard to provide high-quality images, often times spending several hours on each image to get it just right, so it is a little frustrating to see this effort go to waste in a crummy rendering, but there isn't anything I can do about the problem.