Master Thesis - use of cover illustration
I'd like to provide a counter to the "content is what matters" advice. Of course the content is what it's all about, but I think that a great many people in academia underestimate the importance of presentation. 

Look at the way people decide whether or not to read a paper. They look at the title. If that suggests that it's interesting to them, they read the abstract. If that's interesting enough they read the introduction and maybe the conclusion, and so on. That means that of the 4000 or so words in the paper, the five that make up the title are the most important. You're fully justified in spending as much time on the title as on 1000 words of the description of your method. The presentation of your paper is not just gloss, it determines the size of your audience. If your presentation is bad, your results have to be really exceptional to find even a small readership.

This translates directly to your Master's thesis. A nice cover shows that you care. What's more, the largest group of people that will come into contact with it, will only pick it up, look at it and put it back down again, without even really reading the title. Your cover illustration is the only opportunity you have of communicating with them and enticing them to read the title properly. From that, the summary, and from that the first paragraph of your introduction and so on.

Don't mistake presentation for gloss. Academia is all about communication, and communication is worrying about the details.