This might work -
In the theme-globals.php file, find the following line:
$s5_loop_tags['thumbnail'] = '<img alt="%post_title%" style="position:relative;float:left;padding-right:10px;" src="%thumb_path%" />';
Change it to:
if(!is_singular()){
$s5_loop_tags['thumbnail'] = '<img alt="%post_title%" style="position:relative;float:left;padding-right:10px;" src="%thumb_path%" class="PostThumbnailSmall"/>';
}else{
$s5_loop_tags['thumbnail'] = '<img alt="%post_title%" style="position:relative;float:left;padding-right:10px;" src="%thumb_path%" class="PostThumbnailLarge" />';
}
That by itself won't appear to do anything, but it
will add a different CSS class to the image based on if it's an individual post/page or a list of posts. All you'll have to do after that is add .PostThumbnailSmall{} and .PostThumbnailLarge{} to the bottom of style.css and add actual CSS to make it look the way you want.
If you don't know enough CSS to get it loking the way you want, I recommend checking out the
Codecademy HTML and CSS course to learn the basics of how HTML and CSS work; it's free, and if you combine that with
Firebug, you should be able to figure a lot of things out on your own. Custom coding (including CSS) is not covered by support, so if you need additional help, I suggest checking out the
Hire-a-Coder area and posting a help request.