How to capture images transferred via HTTP?

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http://unix.stackexchange.com – I have multiple browsers and I want save all images with size more than 30KB and save to specified address. What is best way? listen to tcpdump and read the content, and then save to a file, or use something like squid I think tcpdump is better but I don't know how to find out whether the content is an image and of what size (and other attributes) to save it in a file. (HowTos)