Hi Peter and welcome to our :),

One way that you could do it is to use this code below.

@{
    var randomImages = Umbraco.TypedMedia(1064).Children.RandomOrder().Take(1);
    foreach (var image in randomImages)
    {
        <img src="@image.Url" alt="@image.Name" />
    }
}

Where the ID is the ID of folder from the media section where you want the image taking from.

If you want the user to pick a folder on a content node using the media picker then you need to use this code.

@{
    if(Model.Content.HasValue("mediapickeralias")){
        var mediaItem = Umbraco.TypedMedia(Model.Content.GetPropertyValue("mediapickeralias")); 

         foreach (var image in mediaItem.Children.RandomOrder().Take(1))
        {
            <img src="@image.Url" alt="@image.Name" />
        }
    }   
}

Remember to change the mediapickeralias so it match your alias for the media picker on the document type.

Hope this helps,

/Dennis