Aw: Category Images
The above certainly works if you want to completely remove the category image from BOTH category_default.php AND mainpage.php
However if you want a conditional removal of images based on if you have a Category image or not then I found the below code to work. I have left the original code in between <!--- ---> as it is always good to leave it for reference.
<!---<a href = "<?php print $category->category_link;?>"><img class = "jshop_img" src = "<?php print $this->image_category_path;?>/<?php if ($category->category_image) print $category->category_image; else print $this->noimage;?>" alt="<?php print htmlspecialchars($category->name)?>" title="<?php print htmlspecialchars($category->name)?>" /></a>--->
<a href = "<?php print $category->category_link;?>"><img class = "jshop_img" src = "<?php print $this->image_category_path;?>/<?php if ($category->category_image) print $category->category_image; else print '';?>" alt="<?php if ($category->category_image) print htmlspecialchars($category->name); else print ''?>" title="<?php print htmlspecialchars($category->name)?>" /></a>