```
Imports System.IO

Public Class Form1
    Private Sub btnSubmit_Click(sender As Object, e As EventArgs) Handles btnSubmit.Click
        Dim directoryPath As String = txtDirectoryPath.Text
        If Not String.IsNullOrEmpty(directoryPath) Then
            RenameImageFiles(directoryPath)
        Else
            MessageBox.Show("Please enter a valid directory path.", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If
    End Sub

    ''' <summary>
    ''' Renames all image files in the specified directory using the format "image-####", where #### is a sequentially increasing number.
    ''' </summary>
    ''' <param name="directoryPath">The path of the directory containing the image files.</param>
    Private Sub RenameImageFiles(directoryPath As String)
        If Directory.Exists(directoryPath) Then
            Dim imageExtensions As HashSet(Of String) = New HashSet(Of String)({".jpg", ".jpeg", ".png", ".gif", ".bmp", ".tiff"})
            Dim files As FileInfo() = New DirectoryInfo(directoryPath).GetFiles()
            Dim counter As Integer = 1

            For Each file As FileInfo In files
                If imageExtensions.Contains(file.Extension.ToLower()) Then
                    File.Move(file.FullName, Path.Combine(directoryPath, "image-" & counter.ToString("D4") & file.Extension))
                    counter += 1
                End If
            Next
        Else
            MessageBox.Show("The specified directory could not be found. Please enter a valid directory path.", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If
    End Sub
End Class
```
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