If the vector/ pixel based (rastor) files are of concern, then you should do it in inDesign or Illustrator as suggested. If not, photoshop is fine. As mentioned, the free transform (cmd T -keyboard shortcut) will allow you to rotate the text to the sideways position you want. While you are doing this, if you hold down the shift key it will help you position the text perfectly at 90 degrees. Added bonus, change the opacity in layers palette/window to make it a bit less distracting to the image while still maintaining the credit. Could also add a layer style.. even cooler.
Thanks very much… High School Media Teacher, multiple Macs (tiger and snow leopard- mac pros, imacs, emacs) and PCs