There is no quick way to say to  Azure Container to only return specific file formats in a listing. You have to filter them after getting the full listing.

using System.IO; 

            IEnumerable fileList = container.ListBlobs();
            List files = new List();

            foreach (var file in fileList)
            {
                var e = Path.GetExtension(file.Uri.ToString());
                if (e.Equals(".png") || e.Equals(".jpg") || e.Equals(".jpeg"))
                {
                    files.Add(file);
                }
            }