Dreamweaver, using behavior appear in new window

MY PARTNER AND I used Dreamweaver to style this site. On two pages i create a gallery associated with images. How they work is you visit a little thumbnail with the image, and a much larger image appears in the new window. Well sometimes as soon as that image pops up inside the other window it appears behind the main window that’s already open so the viewer doesn’t realize that it has sprang up until soon after they’ve closed out the principle window. But the actual catch is, it doesn’t happen everyday. I’ve tried that on 6 numerous computers, and some reopen in front, and on others open behind main windowpane. Is there something i’ll do to alter this Please aid.