Images for the web should not be at 300 dpi or ppi. The file size would be
too large and they would take forever to load. Photoshop Elements has a Save
for the Web feature, so I would imagine that Photoshop also has that. Use
that to produce your web images at 72 dpi. Also shouldn't your banner be
more like 800 X 75 pixels? If you are truly making the image 600 pixels
high, then perhaps you are also overlapping design elements that are
converting the image and that other element into a new, combined lower
resolution image. Run the design checker tool, and it might give you some
insight into your problem.

DavidF

dude your banner is as tall as my last page