About the error:
The error you are getting could be due gsdll32.dll could not be found or a wrong Ghostscript version installation you used. For a 64bit system you need to install 64bit Ghostscript library which has gsdll64.dll. If you compile your application for AnyCPU platfrom target, on a 64bit system it will run as 64bit process and you will need gsdll64.dll. If you compile your application as x86 and run it on a 64bit system, your application will run as 32bit process and you can use gsdll32.dll. When you use DllImport make sure that dll your are trying to call is in a same (bin) folder your applicatione executes or it can be in windows\system. If you want custom dll location, you can use full path in a DllImport ([DllImport("C:\Program Files\gs\gs9.14\bin\gsdll32.dll", EntryPoint = "gsapi_new_instance")]
) which is normally not recommended.
Why dont you simply use Ghostscript.NET library. It's a well tested native Ghostscript library wrapper which will allow you to do what you need and it's compatible with both x86 and x64 Ghostscript libraries.
Sample code that shows you how to rasterize pdf to image is here: https://ghostscriptnet.codeplex.com/SourceControl/latest#Ghostscript.NET/Ghostscript.NET.Samples/Samples/RasterizerSample.cs
Try different (lower) values with "desired_x_dpi" and "desired_y_dpi" and output image will be smaller.