If SilverRaven's suggestion doesn't work out for you, here is a cheap novelist's trick you might try.
That image is the original cover for my second book before I purchased the current, commissioned cover.
Like suggested in the earlier, I picked out a public domain image. Specifically, I like to browse NASA's Astronomy Picture of the Day archive. Almost half of those astronomy pictures came directly from NASA and are released for free public use -- and they are gorgeous. WikiCommons is a great source of beautiful artwork which has outlived their copyrights and are uploaded in high quality. ...or just search Deviant Art for Creative Commons pieces. If you use a CC piece, remember to fulfill the terms of the license which usually just means acknowledging the artist. These are your backgrounds. Now pick out an interesting font from Blambot.com or FreeFonts. Throw a colorful title over that using software you already have like MS Paint or GIMP or OpenOffice Writer. Within half an hour and next-to-no-skill, you can create your own professional looking original cover that will work well until you get that commissioned piece.