Yeah, first choose "Insert link" and then after pressing OK, choose "Insert Image".
Also remember not to use "enter" between the different URL tags if you want the images side by side on the same row. I would recommend using one "space" between the tags so there will appear a small gap between the pictures. Like in my signature. (I'm using Firefox browser... +"Remove It Permanently" add-on to block some adds)
And also note, that sometimes after adding a new thread, this forum may change the showthread.php's number for some crazy reason. I think I had to change my last thread's URL at least two or three times... Otherwise the link connected to the picture in my signature stopped working...