از این استفاده کن برا من کار کرد!
FolderBrowserDialog fbd = new FolderBrowserDialog();
fbd.ShowNewFolderButton = false;
if (fbd.ShowDialog() == DialogResult.OK)
{
string path = fbd.SelectedPath;
// string Pattern = "*.jpg||*.gif|*.bmp";
var filePaths = Directory.GetFiles(path, "*.*", SearchOption.AllDirectories)
.Where(s => s.EndsWith(".gif") || s.EndsWith(".jpg") || s.EndsWith(".bmp"));
foreach (var s in filePaths)
listBox2.Items.Add(s.ToString());
}