Assuming there are no other values imagebox-*you want to exclude:
$("a[rel^='imagebox-']").colorbox({slideshow:true});
Speaking of which, it is best to avoid attribute selection, and as a rule, it is best for you not to restructure your problem in order to provide a better solution. How could you use classes:
<a href="..." class="imagebox teen"> ...
Note the space: two classes are assigned. So you can do:
$("a.imagebox")...
for all of them. Or:
$("a.imagebox.teen")...
only for one of them.
Remember that you can add multiple selectors with a comma:
$("a.class1, a.class2")...
, , 1 OR class2 :
$("a.class1.class2")...
, , , 1 2.