What causes browsers to render images poorly?

I'm creating a simple html help file from a Word document. Images in the file appear very blurred when the file is viewed a browser. I've tried Firefox and IE8. The image itself is fine. I've also tried saving the image as jpg instead of gif but that didn't help at all.

Below are some screenshots showing a small part of the image as it looks if I view the image in Irfan and as it looks in Firefox. By the way the original here is from the copy of the image that Word created when I saved the file, and that's the image that is pointed to in the html code.

https://chrisdancer.opendrive.com/files?53743686_g...

https://chrisdancer.opendrive.com/files?53743685_e...