Vb.net – How to move an element by clicking and holding the object with the mouse

I am working on a simple program which requires me to be able to select a picture box and move it to a new location by dragging it with my mouse. This is all the relevant code I have come up with currently. However, when I run the program it tries to move to where I want it to go then it seems to revert back to its previous location.

Edit: it is in a container. If this is of any relevance.

Variables

Dim startx As Integer
Dim starty As Integer
Dim endy As Integer
Dim endx As Integer
Dim finalx As Integer
Dim finaly As Integer
Dim mdown As Boolean
Dim valx As Boolean
Dim valy As Boolean

Code to make image move

    Private Sub picbox_MouseDown(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles picbox.MouseDown
        startx = MousePosition.X
        starty = MousePosition.Y
        mdown = True
        valx = False
        valy = False
    End Sub


Private Sub Main_MouseMove(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseMove

End Sub

Private Sub picbox_MouseMove(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles picbox.MouseMove
    'Check if mouse=down
    If mdown = True Then
        endx = (MousePosition.X - Me.Left)
        endy = (MousePosition.Y - Me.Top)

        If valy = False Then
            starty = endy - sender.top
            valy = True
        End If
        If valx = False Then
            startx = endx - sender.left
            valx = True
        End If
        sender.left = endx - startx
        sender.top = endy - starty


    End If
End Sub

Private Sub picbox_MouseUp(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles picbox.MouseUp
    mdown = False
    valx = False
    valy = False

End Sub