Low memmory. What to do?

[Norkis]

Hey, I have made app, which repaints and changes wallpaper after some period. But something goes wrong. Every time then function is called ~7 MB of memory are "ate". How to free it after doing job? I tried dispose, but nothing.

Public Function Paint_text(ByVal txt As String) As Bitmap
 
Dim bm_source As New Bitmap(Image.FromFile("C:\piv.bmp"))
Dim bm_dest As New Bitmap(CInt(bm_source.Width), CInt(bm_source.Height)
Dim gr_dest As Graphics = Graphics.FromImage(bm_dest)
gr_dest.DrawImage(bm_source, 0, 0, bm_dest.Width, bm_dest.Height)
 
Dim mybrush As New Drawing2D.LinearGradientBrush(ClientRectangle, FontDialog1.Color, FontDialog1.Color, Drawing2D.LinearGradientMode.Horizontal)
 Dim myFont As New Font("Verdana",  12, FontStyle.Regula)
gr_dest.DrawString(txt, myFont, mybrush, New RectangleF(x1 , y1 , x2 - x1, y2 - y1)
Paint_text = bm_dest
 
        gr_dest.Dispose()
        mybrush.Dispose()
        mypen.Dispose()
        myFont.Dispose()
        bm_source.Dispose()
        bm_source = Nothing
        bm_dest = Nothing
 
    End Function

Then I set returned image to picturebox (~3MB). But it's OK. This is not why after every function call memory isn't cleared.