That's a 'how long is a piece of string' type question, Phil/Lil.
If you've saved a picture from a website it might only be 50Kb. If you've taken it on a digital camera, using the default settings, it might be 500Kb. If you've used the highest quality settings, it might be 5000Kb (=5Mb) i.e. some picture take up 100 times the disk space that others do.
Go your 'My Pictures' folder (or wherever else you store your image files). Click on 'View' and 'Details'. You'll then see the sizes of your pictures. Estimate a 'highish average' size, then divide 700,000 by that number. (Actually, I don't like putting more than 650Mb on a disk. Errors seem to happen above that figure. So divide 650,000 by the 'average' size of an image file, to get an estimate of how many you can store on one disk).
Alternatively (and far simpler), when using your burning software, just keep adding files to the list to be burnt, while watching the bar on the screen which indicates the extent to which you're filling up the disk.
Chris