Prior to jQuery 1.7
The following will work:
$('.tabContent img[value="+selectboxvalue+"]').css({border: '1px solid #c10000'});
jQuery 1.7 and later
In 1.7 jQuery changed the syntax to require the attributes
to have quotation around the value:
$('.tabContent img[value="'+selectboxvalue+'"]').css({border: '1px solid #c10000'});