Thanks a lot for the information, I did replace stylesheet_directory with template_directory in my files. I tried :
<?php $path = bloginfo('template_directory');
the_content('<img src="' . $path . '/images/readmore_big.gif"
class="alignright">'); ?>
But it did not work either, unfortunately. I did try your second suggestion but I fear that I haven’t put it properly into the code. I’m quite only an amateur when it comes to php. Here’s what I did with your second suggestion :
<?php the_content('<img src="<?php include (TEMPLATEPATH . \'/images/readmore_big.gif\'); ?>" class="alignright">'); ?>
Didn’t work either, many thanks for your info though. Is it even possible to do what I want to do?