I am sorry but your message is quite confusing. What do you mean by 'when upload success --> convert thumb image :'? Which version of Magick.NET are you using, are you using the .NET 2.0 version? And I don't understand what you are trying to tell me with the gacutil.exe / FTP article.
When you receive the message 'System.IO.FileNotFoundException: Could not load file or assembly 'Magick.NET-x64' it means that your application cannot find the Magick.NET assembly. Did you upload it to your server?
When you receive the message 'System.IO.FileNotFoundException: Could not load file or assembly 'Magick.NET-x64' it means that your application cannot find the Magick.NET assembly. Did you upload it to your server?