Use an ftp client to either download about.php to your desktop, or edit it in place. Make the changes, save and place the file back in the theme folder. Use a plain text editor only. While you are in that file you will notice the reference to the image.
<img src="<?php bloginfo('template_directory'); ?>/images/avatar.gif" class="avatar" alt="" />
<b>The photo and the text can be changed by modifying the about.php file.</b>
So the image is located in the images folder of the roadkill theme. You can; a) upload a new image and change the path in about.php, or... and I like this because it makes the most sense... rename the original avatar.gif to avatar.gif.bak then rename an image of your choice (of the same size, of course) to avatar.gif and upload it the same folder where the original is. Then all you have to do is refresh your browser and your image is in place.
1) access the about.php file and avatar.gif image with an ftp client or your hosts file manager.
2) edit the about information in about.php and save the changes.
3)rename the old avatar.gif and upload your new avatar.gif to the same location