Gridview populating only one row after using ashx page

I have a file upload control in page and storing image file in a session datatable. Then i am databinding to a gridview. The gridview have a image tag with the url to a aspx page that return the image. All works fine except the gridview is populating only one row. The below are the detail:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load Dim dtPicRecHistory As New DataTable() dtPicRecHistory = DirectCast(Session("PicRecHistory"), DataTable) Dim imageid As String = "" If imageid Is Nothing Then Return Else imageid = Request.QueryString("imgID") End If dtPicRecHistory.DefaultView.RowFilter = Nothing dtPicRecHistory.DefaultView.RowFilter = "ImageID='" & imageid & "'" If dtPicRecHistory.DefaultView.Count > 0 Then Dim str As Stre ...