You'll need to re-build the code and then sign it - that's the only way. Maybe the best thing to do would be to directly request that LibTiff.Net provide a signed version of their assembly.
Otherwise, you could disassembly the code (see ildasm at https://login.live.com/login.srf?wa=wsignin1.0&rpsnv=11&checkda=1&ct=1376595663&rver=6.0.5276.0&wp=MCLBI&wlcxt=msdn%24msdn%24msdn&wreply=http%3a%2f%2fmsdn.microsoft.com%2fen-us%2flibrary%2ff7dy01k1.aspx&lc=1033&id=254354&mkt=en-US) but remember that depending on their license, you may be legally forbidden from redistributing it after rebuilding it.