Image flicker when replacing Images through CSS

[Blue Apricot]

I think this is straighforward enough so I don't need sample code, but
I will make an example if someone doesn't understand.

BACKGROUND:
We have built a "skinnable" interface, which uses rollover effects
using simple javascript. Before these images were defined in the HTML
(IMG src and TD Backgrounds), and it worked fine. You moused over a
button, for instance, and it changed immediately.

Now we have switched everything to external CSS -- in order to
facillitate different "skins" easily -- using spacer transparent GIFs
to hold the shape required, and on mouseover changing a background
image that is defined in external CSS. This problem (outlined below)
also exists for changing regular images in addition to the background
images, when they are defined in the CSS.

ISSUE:
Problem is, there is a noticable flicker now, when images are
"replaced". It is as if when you put an img src/url in the CSS it
doesn't cache, whereas using the oldschool method and putting it in the
HTML, it does cache.

Is there a way to eliminate this flicker?
Am I right about img src/url not caching in CSS?

Thanks,
B.A.