Firstly, I'm assuming that the control called 'Graph' is actually a Picturebox.
If so, then
Set bmp=Graph.Picture
will do assign an empty picture object because, contrary to what you might expect, the picturebox's picture property is empty. You either need to add:
Graph.Picture=Graph.Image
prior to the bmp call, or you need to change the bmp call to:
Set bmp=Graph.Image
Secondly, the second parameter of the savepicture call needs to be a String, not a picture object.
So, changing your code to something like:
Private Sub Command1_Click()
Dim bmpPicture As Picture
Dim strFile as string
strFile="myfilename"
xMin = xMin.Text
xMax = xMax.Text
Ymin = Ymin.Text
Ymax = Ymax.Text
Graph.Scale (xMin, Ymax)-(xMax, Ymin)
Graph.Line (xMin, 0)-(xMax, 0)
Graph.Line (0, Ymax)-(0, Ymin)
For H0 = xMin To xMax
For V0 = Ymin To Ymax
Graph.Circle (H0, V0), 0.1
Next V0
Next H0
Graph.Picture=Graph.Image
Command1.Visible = False
xMin.Visible = False
xMax.Visible = False
Ymin.Visible = False
Ymax.Visible = False
Label1.Visible = False
Label2.Visible = False
Label3.Visible = False
Label4.Visible = False
Set bmpPicture = Graph.Picture
SavePicture Graph.Image, strFile
Set UserControl1.Picture = bmpPicture
Unload Form1
End Sub