program.cs

using System;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using LinqToTwitter;

namespace TweetImage
{
    class Program
    {
        static async Task Main(string[] args)
        {
            var text = "Hey @csauer I sent this from a .NET console program! #codemash";
            var filename = "C:\\whatever\\path\\foo.jpg";
            var exists = File.Exists(filename);
            if(!exists)
                throw new FileNotFoundException($"Couldn't find {filename}!");
            await TweetIt(text, filename);
        }

        private static async Task TweetIt(string tweetText, string filename)
        {
            var auth = new SingleUserAuthorizer
            {
                CredentialStore = new SingleUserInMemoryCredentialStore
                {
                    ConsumerKey = "<your consumer key>",
                    ConsumerSecret = "<your consumer secret>",
                    AccessToken = "<your access token>",
                    AccessTokenSecret = "<your access token secret>"
                }
            };

            var twitterCtx = new TwitterContext(auth);

            Media media = await twitterCtx.UploadMediaAsync(File.ReadAllBytes(filename), "image/jpeg", "tweet_image");

            Status tweet = await twitterCtx.TweetAsync(tweetText, new ulong[] { media.MediaID });

            twitterCtx.Dispose();
        }

    }
}