How CONVERT a lat/lon point TO a x/y point ON a world mercator proj image in JS?

Question

Thx to read and think about my problem! :)

I have a map world image like that (mercator proj):

http://www.learnnc.org/lp/multimedia/14596

And I have a lat/lon point to place on it in javascript, I am looking for a solution since long time but impossible to solve it..

I have found this example (first of the page) which do this:

http://www.netzgesta.de/mapper/ (mouse lat/lon position is write at bottom/left map) but impossible to retrieve the function from the source..

So is there someone who got an idea? And I will need to do the opposite, pointer will be draggable and I will need to convet to lat/lon.

thanks Bertrand from france