Theme using full size image as opposed to a thumbnail

I am using a theme which is calling the full size images and then just limiting their size, which means for large images its taking forever to load. This is in the theme file

<?php
					if(have_posts()) {

							 while (have_posts()) {
								the_post(); 

								$t = get_the_title();
								$title_limit=substr($t, 0, 25);
								$permalink=get_permalink();
								$icon='';
								$title='<a href="'.$permalink.'" title="'.get_the_title().'">'.$title_limit.'</a>';
								$readmore='<span class="more-link" href="'.get_permalink().'"> […]</span>';
								if($post->post_content != "" && $post->post_excerpt != ""){
									$excerpt= $post->post_excerpt;
									$description= substr($excerpt, 0, 30);
								}
								else if($post->post_content != "" && $post->post_excerpt == ""){
									$excerpt= $post->post_content;
									$description= substr($excerpt, 0, 30);
								}
								else{
									$excerpt= get_the_excerpt();
									$description= substr($excerpt, 0, 30);
								}

								//$excerpt= string_limit_words($description,50);
								//insert the description in the lightbox if descriptions are disabled
								$top=$imgHeight/2;
								$left= $imgWidth/2;
								$thumbnail= wp_get_attachment_url( get_post_thumbnail_id( $post->ID ) );
								//get the image title
								$data = explode('/', $thumbnail);
								$image_title=explode(".jpg", $data[8]);

								if ( get_post_meta($post->ID, 'lightbox', true)){
										$icon='gallery-video';
										$openLink='<span class="portfolio_image"> ';
										$closeLink='</span>';
										$span_icon='<div id="icon" style="display:none;"><h4><a href="'.$permalink.'"><h4>'.$t.'</a></h4></div>';

								}else{
								$icon='gallery-image';
								$openLink='<span class="portfolio_image">';
								$closeLink='</span>';
								$span_icon='<div id="icon" style="display:none;"><h4><a href="'.$permalink.'">'.$t.'</a></h4></div>';
								}
								$terms=wp_get_post_terms($post->ID, 'portfolio_category');
								$term_string='';
									foreach($terms as $term){
										$term_string.=($term->name).',';
									}
								$term_string=substr($term_string, 0, strlen($term_string)-1);
								$samp=str_replace(" ","-",$term_string);
								$string = str_replace(","," ",$samp);
								$finale= $string." ".$page;	

								//generate the final item HTML
					$html.= '<div class="element element'.$column.' '.$finale.'" data-filter="'.$finale.'">'.$openLink.''.$span_icon.'<img src="'.$thumbnail.'" width="'.$imgWidth.'" height="auto" title="'.get_the_title().'"/>'.$closeLink.'</div>';
				}
						}
			echo $html;;
		?>

[Please use the code buttons when posting code here – as is your code may have been damaged]

I have set an imagesize called “mythumb” up of the size I want in the image file – but every change I have tried to make to the above has failed. Can someone help?