Ah well sorry, if i say something offensive, but the photoshop uncovers itself best when it is made to look more real than edited. What are you creating is good, looks good, we know PS, so we can say that its very nice for a beginner, but in case you really want to create a good effect, just keep in mind one thing, make it look more like natural. That gives the real taste.
Like, what you created in the first pic was good, but it would have been awesome if you could have made it something like the skin has been peeled off and the skull part is visible. That would have really magnificent.
But again, good work.
One Advice: Whenever you get time, try browsing various arts, drawings and pictures (especially the edited ones), it would always keep your spirits high to do something better and will provide immense knowledge towards designing and editing.