First, because for most of us this is a skill we will need to rely on another person to complete this, and the time and money it will take will depend on when we can complete the process of publishing our story.
Second, because we should have at least the outlines of our website determined before our book is published, and the art for the cover of our book is an essential element needed to make our website really stand out and promote our story.
I went ahead and found my artist at http://www.deviantart.com/ Just by going over artists websites and profiles you get an excellent idea of the different types of art available for your story, and normally you can see the going rate of what each artist is charging for the work they are doing. Also,
you have the perfect medium to communicate with them and arrange for the work that you would like for your cover.
Typical practice is for you to explain what you are looking for and a general price to be discussed. Once that has been determined, normally you work with the artist to get a rough sketch of what you would like to be drawn. The artist sends you the rough sketch for your approval to proceed with the final image, and you pay half of the cost at this time. The last half is due at the time the artist has finished the piece to both your satisfaction. You get full rights over the image for your website and book and to do as you like, and they get the option to post their artwork on their website.
Prices vary greatly between artists, so it really is important to do your homework. I did my cover with Leo Black http://dleoblack.deviantart.com/ who was a great artist to work with. He made me feel comfortable with making changes and brought my image to life. Not to mention, I think his artwork is incredible.
Once you have your cover finished for your book. I suggest getting your website created and setup if you do not already have one. I used www.weebly.com for my page. As long as you have your own content ready to go, aka artwork, everything else is drag and drop for the entire design. No other special software is needed and they link automatically with Twitter and Facebook and other websites to automate your posts and help you
establish your web presence.
This is important since you will be doing your own marketing.