Hey,
WordPress' Featured Image function automatically resizes images which are uploaded so they are available at the correct size for placement within the theme. The sizes to be generated are defined by a themes functions.php file.
In BrightBox, 6 image sizes are defined (near the top of functions.php) - this is the code which generates them:
$avia_config['imgSize']['widget'] = array('width'=>48, 'height'=>48 ); // small preview pics eg sidebar news
$avia_config['imgSize']['archive'] = array('width'=>230, 'height'=>80); // medium preview pic for archives
$avia_config['imgSize']['portfolio'] = array('width'=>243, 'height'=>125); // medium preview pic for portfolio
$avia_config['imgSize']['blog'] = array('width'=>574, 'height'=>200); // image for blog posts
$avia_config['imgSize']['page'] = array('width'=>726, 'height'=>250); // image for pages
$avia_config['imgSize']['featured'] = array('width'=>978, 'height'=>99999); // big images for fullsize pages and mainpage slider. no height limitation
There shouldn't be 17 images for each image you upload; there should be the original plus the 6 defined above so a total of 7. If you are getting extra images, have you adjusted the code to include more images, or activated any plugins which affect the images?
James