AFAIK, all the previews in X use a simple 'nearest pixel' algorithm for showing reduced size versions of bitmaps. If your image is viewed at 20%, say, X will use every 5th pixel from the original and ignore the other 4. Hence a large portion of the image can be totally ignored, but this is common practice and normally doesn't matter. More sophisticated algorithms take some sort of average of adjacent pixels to give a better overall appearance, however the preview may not then contain any of the original colours.

Regards - Sean