Download pictures from Reddit
There are few things you can do. Start by encapsulating these lines 


if (imageURL.Contains("gfycat.com"))
{
    imageURL = imageURL.Replace("gfycat.com", "zippy.gfycat.com") + ".mp4";
}

if (imageURL.Contains(".gifv"))
{
    imageURL = imageURL.Replace(".gifv", ".mp4");
}



in their own method. Let's call it FixImageUrl (this is C# 7)

public static string FixImageUrl(string imageUrl)
{
    switch (imageUrl)
    {
        case string url when url.Contains("gfycat.com"):
            return imageUrl.Replace("gfycat.com", "zippy.gfycat.com") + ".mp4";

        case string url when url.Contains(".gifv"):
            return imageUrl.Replace(".gifv", ".mp4");

        default: return imageUrl;
    }
}





string[] splitURL = imageURL.Split('/');
int index = splitURL.Length - 1;
string fileName = splitURL[index];



Then you can simplify this a little bit by using the Last extension:

var fileName = imageURL.Split('/').Last();




It probably doesn't matter here but it's a good habit to always dispose objects that are disposable. This means the WebClient requires special care. 

You also want to know what went wrong so don't swallow the exception. Print either the message or log the entire exception. 

When working with paths you should use the Path.Combine method to avoid any invalid paths like ones with missing \ etc.

try
{
    using (var client = new WebClient())
    {
        client.DownloadFile(imageURL, Path.Combine(userDir, fileName));
    }
}
catch (Exception ex)
{
    Console.WriteLine($"[INFO] ERROR DOWNLOADING FILE: {ex}");
}