These types of visual effects are normally done with Flash... just so ya know that. That allows for a full-screen effect without the heavy download. And either way, this effect is not suitable for dial-up connections. But that's something you, the designer, will have to decide on yourself. Just think about who your target audience will be, and design accordingly. That's the general rule for this case.
The photo you're using right now Doc is 1/2 a meg. That's extremely large. Try a few optimization tests and see if you can get the image(s) down a bit further.
My thoughts to your idea would be to simply create 2 images -- one lit, the other darkened. A mouseOver would switch the 2 images real-time.
The download time though would make the effect not be quick enough to be appreciated. You'd have to "perLoad" the lightened image for the user. But to do that effectively, you'd have to distract them in some way so that the lightened image can be downloaded and ready to swicth immediately.
IDEA: put a flashlight on the desk and have that turned on, and pointing indirectly at the light switch. That would be more appropriate then just having a 'phantom' light shining in from some unknown source.

Or... use light from the windows perhaps...
As for the design measurements...
Designers MUST know that no matter what screen res they design for, they will
always have to subtract at least 30-40 pixels from the width of their design. Browsers have 'chrome' (i.e. borders & scrollbar) that have to be taken into consideration.