That’s HUGE!
I have good news. WordPress handles creating image thumbnails. You can add new sizes or use the thumb/medium/large sizes that are baked in. You just have to call wp_get_attachment_image() with the image size as long as the image are in the media library.
Want to add a new image size for a thumbnail? Call add_image_size() with the width/height and whether or not it should be cropped vs resized.