Post Thumbnail and Image Size

Hi,
I am wondering if anyone can help me out/better understand post thumbnail and image sizes. I’m far from an expert at code, etc., but I have been at trying to understand how this works for over a year.
In my theme, there is a section that displays all my blog posts two at a time, side by side; how can I get these images to display the same dimensions and “line up” with each other, so one pic isn’t way up here and the other way down there? Can you explain this in context with the regular size of the image when clicking on it?
I have set the values as such in functions php:
//FEATURED IMAGE SUPPORT
add_theme_support( ‘post-thumbnails’);
set_post_thumbnail_size( 500, 500, true);
add_image_size( ‘post’,9999,500 );
add_filter( ‘post_thumbnail_html’, ‘remove_thumbnail_dimensions’, 10 );
add_filter( ‘image_send_to_editor’, ‘remove_thumbnail_dimensions’, 10 );
function remove_thumbnail_dimensions( $html ) {
$html = preg_replace( ‘/(width|height)=\”\d*\”\s/’, “”, $html );
return $html;
What am I not understanding? Does every image I make have to be the same dimensions before I even upload it? What’s the point of setting values then in CSS? Help!
Thanks for reading.