(/\ Short video tutorial, summarized into text at the bottom of the linked page.)
And if cropping is not what you wish to do, I am sure that you can also rotate an image/straighten it as well, with the program.
There are other free programs that will do the same thing. (Google is your friend on this note. Additionally, for searching out various tutorials.)
However, am suggesting Photofiltre, since once you get to know your way around ... it has many other functions that are useful when it comes to post processing/editing your photos.
This is a charcoal I am guessing , its terrific. I like what you did with the lighting too. Les has a good point of how to improve on this and I agree with it..Very well done.