You're doing it all wrong:
"<a href=\"register.php?s=$session[sessionhash]&action=signup\">".
"<a onmouseover=\"changeImages('image2', 'image2a')\" onmouseout=\"changeImages('image2', 'image2b')\" ".
"img name=\"image2\" ".
That's not valid HTML _whatsoever_

Make it:
"<a href=\"register.php?s=$session[sessionhash]&action=signup\">".
"<img onmouseover=\"changeImages('image2', 'image2a')\" onmouseout=\"changeImages('image2', 'image2b')\" ".
" name=\"image2\" ".