How to remove the thumbnail on top of my post in Twenty Twelve theme

You will want to change the content-single as well.

the unwanted thumbnails on the post pages are still there.

this is because the single post uses the same content.php as the posts page;

this is the code from the sinlge post:

<!-- Adds the 300x300 thumbnail/featured image -->
        <div class="excerpt-thumb">
            <a href="https://dongyuesu.com/art/portaiture/" title="Permalink to (Portraiture) I Am Who I Think You Think I Am" rel="bookmark">
            <img width="300" height="200" src="https://dongyuesu.com/art/wp-content/uploads/2013/02/artlife-14-300x200.jpg" class="alignleft wp-post-image" alt="artlife-14" />            </a>
        </div>

if you don’t want that in single posts, you need to wrap your code into a conditional statement;
example:

<?php if( !is_single() ) : ?>
YOUR CODE
<?php endif; ?>

<?php if( !is_single() ) : ?>
YOUR CODE
<?php endif; ?>

Thanks so much for the prompt reply!! This code resolves the problem. Thanks so much!

One more related question:
In my category page, the thumbnail is on top of the post title. If I want it to look like my homepage, where the thumbnail is on the left next to the post title, what should I do?

Here is what my category page looks like:
https://dongyuesu.com/art/category/photography/

you only removed the ‘clear’ for the post titles in the posts page;

.blog .entry-title, .blog .entry-header .entry-meta { clear: none; }

extend theat to archive pages as well;

for example, add:

.archive .entry-title, .archive .entry-header .entry-meta { clear: none; }

Thanks again. I have just done what you said, by extending the { clear: none; } to include archive, but things aren’t changed. Besides CSS, could there be somewhere else that I need to fix?

Besides CSS, could there be somewhere else that I need to fix?

always clear the browser cache after changes to the styles, if you want to see the results immediately.
press ‘ctrl f5’ or ‘reload’ the web page a few times (or check your browser’s instructions)

btw:
I see the thumbnails to the left of the content, in the category archive.

Oh yes you are right. The category archive is now showing what I want. Problem solved!
I did refresh my browser a few times but things remained the same though. Then I switched to another computer and I saw the change. You are right that I need to clear the cache; but many other changes do not require clearing cache. That was why I had doubt. Sorry for that.

Thanks again Alchymyth. I saw you quite active in different treads helping people. I am very grateful to your help!