If the machine that's running the script is behind the firewall, you probably won't be able to "defeat the firewall" anyway (except see last paragraph).

IIRC internet explorer will show a red cross i.e. a "red ex" for either unreachable urls AND incorrectly formatted images.

My suggestion to subvert a firewall would be to use a http proxy. Searching for "anonymous proxy" on a www search engine should give you plenty of results.