Picturebox inside a Picturebox

[ca1n3]

I am using this code to move the PictInner inside of PictOuter to a certain point inside of it:

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Private Sub PictOuter_MouseDown(Button As Integer, Shift As Integer, x As Single, Y As Single)
Dim pos As String

pos = Format(x / PictOuter.Width * 100, "0")
PictInner.left = PictOuter.Width * pos / 100
End Sub
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My question is, how can I get the person to drag PictInner, instead of just being able to click somewhere in PictOuter, just like you would drag the sliderbar in a slider.