1) Copy all the pictures into one folder.
2) From the "Actions" palette, select "New Action."
3) Give it a name, then hit "Record."
4) Open a picture from the folder (remember the name).
5) Rotate it 90 degrees.
6) Save and close the picture.
7) Click the "Stop" button in the Actions palette
8) Re-copy the original file back into the folder, overwriting the rotated one.
9) Highlight the action in the Actions palette, hit play, select the folder and go!
I think options for source folder and destination folder are given at the time the Action is run, but if not, the procedure above should open each picture in the folder, rotate it, then save it back to the folder, overwriting the original.
Kind of tedious, I know, but Actions in PhotoShop save much time when used in the right place!