The solution is interesting and may relate to this puzzle. Each image is converted to six bits of information, and this series of six bits are used as index into Base58 charset, "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz". To simplify it, each 6 bits are index how much is each letter away from the beginning of charset - the letter/number "1". This series of 6 bits converts to WIF encoded private key in this manner. If something like this applies to this puzzle also, we are looking for series of 6 bits which begin with 5 since all WIF encoded private keys begin with this number.