Photoshop :: Resizing And Professional Printing

how I should prepare several photos for a professional printing lab.  Photoshop is not entirely new to me, but there are still several concepts I am still mastering and or trying to fully comprehend.I am trying to create some poster-sized images of some co-workers to hang up around the workplace here.  They will replace some older out-dated photos that we already have on the walls.  The older, framed and mounted photos are right at 24"(W) x 36"(L).  That size includes a thick black border around the actual photo. The actual photo measures approximately 15"(W) x 23"(L), for the portrait orientation.
 
What I am looking to do is get some new photos printed to the same specifications as 24" x 36".  My question is, should I change the image size of the photos I took within Photoshop to match those specifications, or allow the lab to use their "Blow-Up" software?
 
I've used Lightroom 4 to crop, and make minor edits to my photos. how I should continue with my photos in Photoshop, in order to create the same black border around the photos as well as add a "Motivational Poster-Look" to a couple... All while not losing resolution when planning to print to such a large size.