Check out your local college or community college. I have taken classes at
SUNY Farmingdale and Nassau Community College. Most of what I have learned
though has been on my own. One very helpful resource has been "Teach
Yourself Photoshop (or Illustrator) in 24 hours" published by SAMS and the
Adobe Classroom In A Book Series is also helpful.
Be careful of courses offered through BOCES. Unfortunately the level of
knowledge varies tremendously among their instructors.