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protected void btnUpload_Click(object sender, EventArgs e)
{
    HttpPostedFile file = (HttpPostedFile)(txtFile.PostedFile);
    if ((file != null) && (file.ContentLength > 0))
    {
        if (IsImage(file) == false)
        {
            // Invalid file type so do something here
            return;
        }
    }


    int iFileSize = file.ContentLength;


    if (iFileSize > 1000000)  // 1MB approx (actually less though)
    {
        // File is too big so do something here
        return;
    }


    // all good do what you need to do
}


private bool IsImage(HttpPostedFile file)
{
    // This checks for image type... you could also do filename extension checks and other things
    // but this is just an example to get you on your way
    return ((file != null) && System.Text.RegularExpressions.Regex.IsMatch(file.ContentType, "image/\\S+") && (file.ContentLength > 0));
}