Use an online photoshop software to add effects and text to one of your own photographs, or an image downloaded from an online website that provides access to free images. (You can find these by typing "free images" into a search engine.)
Type "free e-book templates" into an Internet search engine and pick a template that is suitable to your subject. Take care to make an appropriate match: Choose an image or pattern that complements the subject you have written about. Remember that your e-book may be judged by its cover, so use a template only if you feel that it befits the content of your work.
Trade skills with a graphic designer. If you cannot afford to purchase e-book cover software and you have written a book that is ready for publication, chances are that you have writing skills that you can offer to trade with someone who has graphic design skills. If you do not know of anyone from your immediate social circle, use a social networking site to advertise a "skills swap" request.