Gobs and gobs of tutorials here for most popular image editing programs. This link takes you to the Photoshop collection. For others (PhotoPaint, PaintShop Pro, etc.), click on the applicable link that you'll find in the box about mid-page.
Note: These tutorials span many topics. Although some are photo-art related, quite frankly most are not -- but I thought the resource was worth mentioning for general interest.
Happy reading.
~Danny~