C# - Sql Syntax How To Do A Join

Im trying to retrieve a path name from my Pictures table the pathname is stored under picturepath in my Pictures table I dont know how to join it on to my current sql syntax it uses the UserID which is set by a session.

And im unsure what goes in the commented line for my image url string?

using (OdbcCommand cmd = new OdbcCommand("SELECT Wallpostings FROM WallPosting WHERE UserID=" + userId + " ORDER BY idWallPosting DESC", cn))
{
using (OdbcDataReader reader = cmd.ExecuteReader())
{
test1.Controls.Clear();
while (reader.Read())
{
System.Web.UI.HtmlControls.HtmlGenericControl div = new System.Web.UI.HtmlControls.HtmlGenericControl("div");
div.Attributes["class"] = "test";
//div.Style["float"] = "left";
div.ID = "test";
Image img = new Image();
img.ImageUrl = String.Format("{unsure}", reader.GetString(unsure));
// this line needs to be represented in sql syntax
img.AlternateText = "Test image";
div.Controls.Add(img);
div.Controls.Add(ParseControl(String.Format("{0}", reader.GetString(0))));
div.Style["clear"] = "both";
test1.Controls.Add(div);