123
45
150
90
50
The trick here is to create an image that can be stretched in one direction without distorting the image. The image here is a 20x20 image with a vertical gradient. You could save a bit of transmission time by trimming the image width to a single pixel.
Images used:
bar_gray.png
<img src='bar_gray.png' border='0' height='20' width='123'>