Below is the function I use to process the images from the form, where path is context.Record.GetRecordFieldByAlias("imageAlias").ValuesAsString().

// from https://devblogs.microsoft.com/dotnet/net-core-image-processing/
public string ResizeAndSaveFormFileImage(string path)
{
    string wwwrootPath = _environment.WebRootPath;

    string fullPath = Path.Combine(wwwrootPath, path);

    Uri uri = new Uri(wwwrootPath + path);

    string test = uri.AbsolutePath;

    int maxWidth = 800;
    int maxHeight = 600;
    Image image = Image.FromFile(test);

    int width, height;

    // only resize if the image is too large
    if (image.Width >= maxWidth || image.Height > maxHeight)
    {
        if (image.Width > image.Height)
        {
            width = maxWidth;
            height = Convert.ToInt32(image.Height * maxWidth / (double)image.Width);
        }
        else
        {
            width = Convert.ToInt32(image.Width * maxHeight / (double)image.Height);
            height = maxHeight;
        }
    }
    else
    {
        width = image.Width;
        height = image.Height;
    }

    var resized = new Bitmap(image, width, height);

    string base64String = string.Empty;

    using (MemoryStream memoryStream = new())
    {
        resized.Save(memoryStream, ImageFormat.Jpeg);
        var bytes = new byte[memoryStream.Length - 1];
        memoryStream.Position = 0;
        memoryStream.Read(bytes, 0, bytes.Length);
        base64String = Convert.ToBase64String(bytes, 0, bytes.Length);
    }

    return base64String;
}