The "Picture" is called a
bitmap, and it gets the name from the fact it is saved in BMP format, which doesn't "blur" the image like a JPEG one to save file space. It is made with paint, or another image program, and it is not just any old picture, it must be made from the file "scenariobkg".Have a look in your AoK folder for this file. You cannot make the picture larger or smaller, and you must save it in BMP format.
A common way of making bitmaps is to use cut and pasting from a screenshot. You can take a screenshot in the game, then look in the AoK Folder (or screenshots folder for AoC) to find them.
Paint is the most common program to use, but it does take a surprizing amount of time to get used to for such a simple program. With a bit of work you can get a bitmap that looks almost as good as something made from non-free programs like Paintshop Pro.
I don't think there are any bitmap making articles around for AoK, so have a look at Age of Empires Heaven, there's a great one there by Frank Steidel, just keep in mind there are a few subtle differences (but very few)[This message has been edited by rwilde (edited 04-08-2005 @ 04:21 AM).]