I'm not sure if your web host will allow this but in the php.ini you can set an option to call a fixed function on every single page.
Basically, you would write a function that determines the size of the data it's going to send.
Not an easy task.
The other alternative is to create an image serving script that records the bandwidth it used when sending that particular image.
Do a search on google for bandwidth tracking with php.
Hope this helps.
Cheers.
William Geoghegan
GEOTEK Computer Services
- www.geotekcs.co.uk -