[Cross-linking for reference: https://stackoverflow.com/questions/52523055/how-to-display-an-image-from-dropbox-to-mvc-razor-view ]
Using the GetThumbnailAsync method is the right way to get the thumbnail data for an image file. That works the same way as the DownloadAsync method, for which there is an example here:
https://github.com/dropbox/dropbox-sdk-dotnet/blob/master/dropbox-sdk-dotnet/Examples/SimpleTest/Program.cs#L407
What you do with the thumbnail data is up to you, depending on what you want your app to do exactly, and how. For example, you may need to save it to the local filesystem to be read later, or you may need to return it directly to some other part of your app.
Also, "RootCauses.png" wouldn't be a valid path. File paths should start with a "/", so it would be something like "/RootCauses.png" or "/FolderName/RootCauses.png", etc. You can also get the path from a file metadata object, using the Metadata.PathLower property.