You can do it like that
string[] strFiles = Directory.GetFiles("YOUR PATH");
Now bind the repeater with the strFiles .
<itemtemplate>
<a class="vlightbox1" name="leaf" href="upload/ThumbImage/<%#Eval(FILE_NAME)%>">
<img src="upload/PopupImage/<%#Eval(FILE_NAME)%>" /></a>
</itemtemplate>