Adding a post thumbnail is pretty easy, open content.php, if you're using excerpts on your homepage find this

<?php the_excerpt(); ?>

then add this just above that line

<?php the_post_thumbnail('thumbnail', 'class=alignleft'); ?>

If you're not using excerpts add it above

<?php the_content( __( 'Continue reading <span class="meta-nav">&rarr;</span>', 'twentyeleven' ) ); ?>

If you'd like a specific image size for your post thumbnails you can add something like this to functions.php, change 140, 140 to whatever size you want.

add_image_size('excerpt-thumbnail', 140, 140, true);

Then change

<?php the_post_thumbnail('thumbnail', 'class=alignleft'); ?>

To

<?php the_post_thumbnail('excerpt-thumbnail', 'class=alignleft'); ?>

Then just add a featured image to a post.

Not sure what you're asking for the first two questions, maybe you could explain them a little more.

Adding a post thumbnail is pretty easy, open content.php, if you're using excerpts on your homepage find this

<?php the_excerpt(); ?>

then add this just above that line

<?php the_post_thumbnail('thumbnail', 'class=alignleft'); ?>

If you're not using excerpts add it above

<?php the_content( __( 'Continue reading <span class="meta-nav">&rarr;</span>', 'twentyeleven' ) ); ?>

If you'd like a specific image size for your post thumbnails you can add something like this to functions.php, change 140, 140 to whatever size you want.

add_image_size('excerpt-thumbnail', 140, 140, true);

Then change

<?php the_post_thumbnail('thumbnail', 'class=alignleft'); ?>

To

<?php the_post_thumbnail('excerpt-thumbnail', 'class=alignleft'); ?>

Then just add a featured image to a post. It's best to do this with by creating a child theme.

Not sure what you're asking for the first two questions, maybe you could explain them a little more.