Pixel Sizing Demonstration

This chart provides images of various sizes to establish maximum image size that is compatible with browsers operating in 640x480 pixel resolution mode.

The 600x300 image fills but does not exceed the viewing area of a Netscape 1.2 browser with Toolbar, Location and Directory Buttons invoked.
The 600x400 image essentially fills but does not exceed the horizontal viewing area of a Netscape 1.2 browser with only Location invoked, while slightly exceeding the vertical viewing area.
Not surprisingly, 640x480 exceeds both vertical and horizonal limits.

Adobe, which knows a few things about pixels, sez you should design for 600 pixels wide.

Netscape figures on much smaller images - like about 470 pixels wide:

Yahoo also sez keep it small

But Alta Vista takes the cake for the smallest - 425 pixels

Microsoft also sez 600 pixels [but they swing both ways, since their signature image is actually two images that nest side by side on larger clients, but tastefully nest on top of each other for smaller clients: