It all depends. The easiest way and most reliable is for you to use an .htaccess file.
in the file you would specify a sort of translation, where you would say something like this:
if the user is visiting:
actually redirect the URL them to:
Of course, /vacation-pictures.html has to be a valid page or else your visitors will get a 404. -- BTW are you using WordPress? If so, post back and I'll give you a super easy way to do it from within WordPress.