Loading image to temporaly file internet

ok so here is my intro for my question.

i want to make a website. ill make it in flash and publish it so that it will be transparent. so that i will be able to put a random image in the background using html or css. so my problem is that the image will be very big and i dont want it to load bit by bit when the flash will already be loaded. what i would want is that when the preload is loading the flash website i wound want an other preload that would load the image to the temporaly file internet folder of the user, i would also want that it works on pc and on mac.

is it possible, if yes how.

thank you.

ps: sorry for the errors, english is not my native language.