How to put a caption on a popup

On www.klimatosoof.nl/Thoenes I have a table with thumbnails that link to larger pictures. I have put these larger pictures in popups, BUT I would like to display a caption on these popups. Can this be done in Javascript? Should I edit the following text and how?

ONCLICK: window.open('/klimafiles/images/Fig. 1.png','','width=450,height=310,left='+(screen.availWidth/2-225)+',top='+(screen.availHeight/2-155)+'');return false;

Many thanks

TR