Any Way To Compute How Much Data Downloaded?

I am working on implementing a carousel-style switcher (using a pre-built script dependent on the MooTools library) on the home page of one of my Web sites. The script displays a series of thumbnail images. When the user clicks on one of those thumbnails, an AJAX request returns the following items, which are then switched out using javascript:A background image (usually between 50 and 100 kilobytes)A foreground image (usually between 10 and 20 kilobytes)A header for the promotional item (strictly text)The body text of the promotional item (text and HTML code, generally no more than 200 characters of text)I have all of that working properly.

When the user clicks on the thumbnail, the AJAX function executes. Then, I use javascript to invoke a new Image() object to load the background image. Once that's completed, I invoke another Image() object to load the foreground image. Once both requests are fully loaded, I switch out the information. While they are working, I display an animated gif that indicates the information is loading. However, if at all possible, I would like to implement a progress indicator rather than just a simple animated gif. Because of the size of the background images, it can sometimes take between 10 and 15 seconds to load on a dial-up connection (even longer if the user's connection is especially slow or overloaded).

That's a long time to sit and stare at a "loading" image, especially if there's no way to indicate whether it's going to actually do anything or if it's just completely stalled. I am already sending the filesize of the background image and the foreground image to my javascript using the AJAX request, so if I need to use that somewhere in my script, I can easily do that. However, I've hit a roadblock in trying to figure out if there's a way to determine how much the user has actually downloaded successfully. From most of my research, it does not seem possible (due to security issues) to determine any sort of file size on the user's local computer using javascript.

However, the majority of the information I've found on the subject seems to relate to the opposite type of situation (users uploading files rather than downloading). The few examples I've been able to find for "download indicators" seem to simply count the total number of images being loaded, then divide 100 by that count. Then, for each image that loads, it adds that percentage to the indicator. Since I will always be loading two images, that means that my download indicator would always display 0%, 50% or 100%, which isn't much more helpful than the animated "loading" gif I'm already using. Is there any way to use javascript to determine how much (percentage-wise or in bytes) of a file a user has downloaded from the server into their cache?