Mastering Photoshop won't help you master design, it will only help you make extremely tacky, filter-laden graphics so typical of amateurs.
Photoshop is a tool. Knowing how to put a brush to a canvas wont guarantee that you paint awesome pictures. You need to know about colors, proportions and composition, along with lots of practice AND knowledge of the tools, in order to make great designs.
If you really want to get better at design, and don't want to spend money, check out some good design blogs like Smashingmagazine, Webdesignerdepot, UXbooth, Designchapel, Designmeltdown, Designinformer, Noupe. Those are the first that came at thet op of my head; there are MANY more.
Source: I have a masters degree in Media System Design