Assign the alt tag on dynamic images

Hi everyone,
I have the following code (C#) which i use to dynamically get the images from a folder and bind them in a repeater control
The code works fine but the problem is that the rendered HTML will not validate because the images are missing the alt tag
Is there a way to modify the code or something that will add the alt="" to every image in the control?
Thanks
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protected void Page_Load(object sender, EventArgs e)
    {
        ListImages();
    }
    private void ListImages()
    {
        string imgDir = Server.MapPath("~/images/sponsors");
        DirectoryInfo di = new DirectoryInfo(imgDir);
        Repeater1.DataSource = di.GetFiles();
& ...