Topic: Exagerated highlights & tonal range in JPG's?

Here's my dilemma- I am working on a new web site, and I want to update my Soundslides intro slides on all my slideshows. Problem is, I want a very subtle gradient from dark gray in the center to black or almost black at the borders. When I import a jpg that was created in PS, the subtle dark gray is no longer subtle AT ALL. The barely-there gradient becomes very pronounced.

A) Why is this happening, why does the jpg become brighter and more pronounced in the tonal range?

B) Is there any way to avoid it???

Here is an example of the exagerated effect:


This of course is not a Soundslides show, but the effect is the same. I have noticed in the past that Soundslides shows do seem considerably brighter than the original image. I never minded particularly until I was trying to create something very subtle. 

Any help or response is much appreciated!