You could use an online database of Captchas information (hosted on a server), or sharing Captchas information through P2P, including for example a communication system (Jabber?) To ask other users if they already have decoded the captcha correctly (this could be done without user interaction), instead of using the images could be obtained the hash (MD2, MD4, MD5, SHA1, snefru, Haval, tiger, whirlpool, ripemd) and this work way to save space on disk either in the bandwidth.

No. Captcha is usually binded to IP and it has time limitation.

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