How to get rid of pixelated edges of a jpg? - vb.net

Question

For my student project, I’ve a vb.net code snippet that downloads a jpg file from a website and then sets all its pixels, which are almost white, entirely white:
Dim oColor As Color
Dim bRed As Byte
Dim bGreen As Byte
Dim bBlue As Byte
Dim imgTemp As Image = Image.FromFile("C:\DownloadedImage.jpg")
Dim bmpTemp As New Bitmap(imgTemp.Width, imgTemp.Height, Imaging.PixelFormat.Format32bppArgb)
Using gfx As Graphics = Graphics.FromImage(bmpTemp)
gfx.SmoothingMode = Drawing2D.SmoothingMode.AntiAlias
gfx.CompositingQuality = Drawing2D.CompositingQuality.HighQuality
gfx.InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic
gfx.PixelOffsetMode = Drawing2D.PixelOffsetMode.HighQuality
gfx.DrawImage(imgTemp, 0, 0)
End Using
For x = 0 To bmpTemp.Width - 1
For y = 0 To bmpTemp.Height - 1
oColor = bmpTemp.GetPixel(x, y)
bRed = oColor.R
bGreen = oColor.G
bBlue = oColor.B
If bRed <= 254 AndAlso bRed >= 254 - 20 Then '20 = White Tolerance'
bmpTemp.SetPixel(x, y, Color.White)
End If
If bGreen <= 254 AndAlso bGreen >= 254 - 20 Then
bmpTemp.SetPixel(x, y, Color.White)
End If
If bBlue <= 254 AndAlso bBlue >= 254 - 20 Then
bmpTemp.SetPixel(x, y, Color.White)
End If
Next
Next
Dim newForm As New Form With {.BackgroundImage = bmpTemp, .TransparencyKey = Color.White, .BackgroundImageLayout = ImageLayout.Zoom, .BackColor = Color.White}
newForm.Show()
The bitmap created in this way is then set as the BackgroundImage of a form whose TransparencyKey property is white. The result is not a complete failure, but the edges of the isolated object are still white and pixelated:
The result in two different sizes (left) and what I'm trying to achieve (right)
I’m now looking for a way to get rid of those ragged edges, something like Photoshop’s “Refine Edge” function.
Many thanks in advance!