An example command that sends a random image from a predefined array each time it's used.

randomimage.cs

// define a static array of paths that we'll choose the image from
// we make it static so it isn't reinstantiated unnecessarily each time we run a command from this module
private static string[] _imagePaths = new[] { "images/hackerman.jpg", "images/computerman.jpg", "images/polarbear.png" };

[Command("image")]
[Alias("picture")]
[Summary("Sends a random image.")]
public async Task ImageAsync()
{
    // using a new instance of the Random class we randomize a path with an index from 0 to x - 1, where x is the amount of paths in the array 
    var image = _imagePaths[new Random().Next(_imagePaths.Length)];
    
    // we send the image
    await Context.Channel.SendFileAsync(image);
}