I have been running into some BadImageFormatException’s lately and I just discovered the reason today. I found out that I needed to change my targeted platform to the same platform as the assembly was build for, that’s it. Simply right-click the project in Visual Studio -> Properties, Build -> Platform target: [platform of assembly]
It solved the same issue I had with LINQPad, I rebuild my assembly and now it could be loaded. I run Windows 7 x64 so I rebuild my assembly to x64.