I found this website with a few tutorials and honestly found them to be useful for photoshop manipulations. Videos can be found at:
The info may already be known by most, but if you find even one topic that you had not mastered yet, then it may be worth while. Be sure to check it out, you know the old saying "you learn something new everyday", well... here's your chance.
Also be sure to check out my overview of microstock sites, with pros and cons for each at:
The page is especially geared towards photographers trying to get into the microstock world