Sometimes it’s preferable to have a button instead of text to allow the reader to jump to the rest of a post. Following is the code to replace the text with an image:

<?php the_content('<img src=
"' . get_bloginfo('template_directory'). '/images/readmore.png"
alt="read more" title="Read more..." />'); ?>

Obviously, replace /images/readmore.png with your image directory and file name.

(This is slightly different from the instructions in the WordPress Codex, which did not work for me.)