No you can't do that as it's illegal and you'll find yourself banned from these forums if there is any further discussion on downloading images. It wouldn't work anyway, as it is still a copied image and wouldn't boot on the PS2 as it isn't an original disc. Take this as a friendly warning...
Now to answer the other part of your question, it works by fooling the PS2 into thinking that an original is inserted. You put the disc in, and it boots. You then need a set of 'keys' to cover the drive sensors to make the console believe the tray is still shut. This way, you can open the tray, swap out the disc and the software loads the game. However, the discs are not the best solution, swapping quickly becomes a pain in the arse and many games have to be patched etc to work properly. The best solution is easily a proper modchip, such as the Matrix Infinity if you are interested in using backups.
So no, it isn't just the disc which makes the backups load, and by the time you've bought the discs, keys, slide card etc you may as well have bought a proper chip and had it installed. If you're on a budget, try a Duo3. This offers direct boot of all backups / imports etc at a great low price, which is a whole lot better than any swap magic disc and key set...
Hope that helps...
LUFCfan
