public static bool PostImage()
{
    FileInfo imageFile = new DirectoryInfo("Images").GetFiles("*.jpg")[0];
    var api = new VkApi();
    api.Authorize(4857486, "Логин", "Пароль", Settings.All);

    long groupId = 100; // ИД ГРУППЫ
    UploadServerInfo uploadServer = api.Photo.GetWallUploadServer(groupId);
    var uploader = new WebClient();
    byte[] uploadResponse = uploader.UploadFile(uploadServer.UploadUrl, imageFile.FullName);
    string jsonResponse = Encoding.UTF8.GetString(uploadResponse);

    JObject jsonObject = (JObject)JsonConvert.DeserializeObject(jsonResponse);
    string hashParameter = jsonObject["hash"].ToString();
    string serverParameter = jsonObject["server"].ToString();
    string photoParameter = jsonObject["photo"].ToString();
     
    ReadOnlyCollection<Photo> photos = api.Photo.SaveWallPhoto(
        photoParameter,
        uploadServer.UserId,
        groupId,
        long.Parse(serverParameter),
        hashParameter);

    var attachments = new List<MediaAttachment>();
    attachments.AddRange(photos);
    long wallPostResult = api.Wall.Post(groupId, false, true, "", attachments, null);

    return true;
}

Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Imports System.IO
Imports System.Net
Imports Newtonsoft.Json
Imports Newtonsoft.Json.Linq
Imports VkNet
Imports VkNet.Enums.Filters
Imports VkNet.Model.Attachments
Imports VkNet.Model

Public Shared Function PostImage() As Boolean
    Dim imageFile As FileInfo = New DirectoryInfo("Images").GetFiles("*.jpg")(0)
    Dim api As New VkApi()
    api.Authorize(4857486, "Логин", "Пароль", Settings.All)

    Dim groupId As Long = 100 ' ИД ГРУППЫ
    Dim uploadServer As UploadServerInfo = api.Photo.GetWallUploadServer(groupId)
    Dim uploader As New WebClient()
    Dim uploadResponse() As Byte = uploader.UploadFile(uploadServer.UploadUrl, imageFile.FullName)
    Dim jsonResponse As String = Encoding.UTF8.GetString(uploadResponse)

    Dim jsonObject As JObject = DirectCast(JsonConvert.DeserializeObject(jsonResponse), JObject)
    Dim hashParameter As String = jsonObject("hash").ToString()
    Dim serverParameter As String = jsonObject("server").ToString()
    Dim photoParameter As String = jsonObject("photo").ToString()
     
    Dim photos As ReadOnlyCollection(Of Photo) = api.Photo.SaveWallPhoto( _
        photoParameter, _
        uploadServer.UserId, _
        groupId, _
        long.Parse(serverParameter), _
        hashParameter)

    Dim attachments As New List(Of MediaAttachment)()
    attachments.AddRange(photos)
    Dim wallPostResult As Long = api.Wall.Post(groupId, False, True, "", attachments, Nothing)

    Return True
End Function