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You can compare the file size.
Use GD and save in your PC just one image (original, intermediate and thumb), then use Image Magick and save the same image in you PC.
Compare them and see if it is a big diffrence.

Topics you may need: 

Small database ( make you database smaller) for less server resource: http://forum.coppermine-gallery.net/index.php/topic,77935.0.html

Hotlinking to redirect to the page that contains the image (more visitors to your gallery) : http://forum.coppermine-gallery.net/index.php/topic,76683.0.html