How to make Thumbnail Posts responsive

Hi there!

A pair of years ago I ordered a web designer to build a WordPress Based Web. (Here you have it http://www.peluqueriamartinez.com) The case is that some weeks ago I decided that it was time to update it a little bit, but the person who design my web left me in the lurch and I have to do it all by myself. My level of CSS, html and php is really really poor. I have done all the changes I wanted to, except one thing. Please, I need help with that.

In my homepage you will find the excerpts of the last two posts. I wanted to display the featured image just below every excerpt. Ok, I did it, but now I want them to have the same width as the paragraphs of every excerpt. That’s where I have the problems. I’ve used plugins such as “Quick Featured Images” and “Regenerate Thumbnails”, but when I shrink my window everything shrinks eccept those thumbnails. How can I make them responsive?

Here’s the peace of code I use to display my post thumbnails:

<h2 class="titHomeNoticias">Últimas publicaciones</h2>
			 <ul id="homeNoticias">
			 <?php
			 	$args = array( 'numberposts' => '2' );
			 	$recent_posts = wp_get_recent_posts( $args );
			 	foreach( $recent_posts as $recent ){
			 		echo '<li><a href="' . get_permalink($recent["ID"]) . '" title="Look 					'.esc_attr(	$recent["post_title"]).'" >' .   $recent["post_title"].'</a>
			 		<p>'.$recent["post_excerpt"].'</p>'.
			 		'<figure>'.get_the_post_thumbnail($recent["ID"], "thumb").'</figure>'.
			 		'</li>';

				}
			 ?>

Can anybody help me? I’m learning those things, please, be understanding with me…

Thanks!