Hi nauna,

For resizing refer below article.

Resize image without loosing image quality (quality loss) in ASP.Net

Use aspose imaging library to convert to webp format.

Add Aspose.Imaging dll in your project or install from nuget.

Install-Package Aspose.Imaging -Version 21.2.0

The use the below code to convert to webp format.

HTML

<asp:FileUpload ID="fuUpoload" runat="server" />
<asp:Button OnClick="OnUpload" Text="Upload" runat="server" />

Code

C#

protected void OnUpload(object sender, EventArgs e)
{
    if (fuUpoload.HasFile)
    {
        string[] allowedImageTypes = new string[] { "image/jpeg", "image/png" };
        if (allowedImageTypes.Contains(fuUpoload.PostedFile.ContentType.ToLower()))
        {                
            string normalImagePath = System.IO.Path.Combine(Server.MapPath("~/Files"), fuUpoload.PostedFile.FileName);
            string webPFileName = System.IO.Path.GetFileNameWithoutExtension(fuUpoload.PostedFile.FileName) + ".webp";
            string webPImagePath = System.IO.Path.Combine(Server.MapPath("~/Files"), webPFileName);
            fuUpoload.PostedFile.SaveAs(normalImagePath);
            var document = Aspose.Imaging.Image.Load(normalImagePath);
             Aspose.Imaging.ImageOptions.WebPOptions options = new Aspose.Imaging.ImageOptions.WebPOptions();
            document.Save(webPImagePath, options);
        }
    }
}

VB.Net

Protected Sub OnUpload(ByVal sender As Object, ByVal e As EventArgs)
    If fuUpoload.HasFile Then
        Dim allowedImageTypes As String() = New String() {"image/jpeg", "image/png"}
        If allowedImageTypes.Contains(fuUpoload.PostedFile.ContentType.ToLower()) Then
            Dim normalImagePath As String = IO.Path.Combine(Server.MapPath("~/Files"), fuUpoload.PostedFile.FileName)
            Dim webPFileName As String = IO.Path.GetFileNameWithoutExtension(fuUpoload.PostedFile.FileName) & ".webp"
            Dim webPImagePath As String = IO.Path.Combine(Server.MapPath("~/Files"), webPFileName)
            fuUpoload.PostedFile.SaveAs(normalImagePath)
            Dim document = Aspose.Imaging.Image.Load(normalImagePath)
            Dim options As Aspose.Imaging.ImageOptions.WebPOptions = New  Aspose.Imaging.ImageOptions.WebPOptions()
            document.Save(webPImagePath, options)
        End If
    End If
End Sub