How to increment a counter slowly while mouse down on picture box?

Try using the Async Await Task-based Pattern. Trap the PreviewMouseLeftButtonDown and PreviewMouseLeftButtonUp events. I would not use MouseLeftButtonDown as it is handled by the Button Click event and will not be trapped by your method. I have just noticed that your button is not a button but a Picture Box but the idea is exactly the same

Imports System.Threading

Class MainWindow
    Dim cts As CancellationTokenSource

    Dim counter As Int32
    Private Async Sub PicButton_OnMouseDown(sender As Object, e As MouseEventArgs)
        ' Instantiate the CancellationTokenSource.
        cts = New CancellationTokenSource()
        Try
            While (True)
                'The CancellationTokenSource causes the loop to be broken
                'by throwing  an  OperationCanceledException
                'when the CancellationTokenSource.Cancel() method
                'is called in the mouseUp handler
                ' Asynchronous Delay 300millisecs
                'Ui thread is not blocked
                Await Task.Delay(300, cts.Token)
                counter = counter + 1
            End While

        Catch ex As OperationCanceledException
            cts = Nothing
            'Reset the counter here, if necessary
        End Try
    End Sub
    Private Sub PicButton_OnMouseUp(sender As Object, e As MouseEventArgs)
        If cts IsNot Nothing Then
            cts.Cancel()
        End If
    End Sub