This ia a rough example of making the image kirbytext tag support PhotoSwipe and automatically generate thumbnails. (Works - but not tested entirely through)

image-autothumb-kirbytexttag.php

<?php

// Custom image kirbytag highjack
// This file should reflect the image tag functionality found in /kirby/extensions/tags.php
kirbytext::$tags['image'] = array(
    'attr' => array(
        'width',
        'height',
        'alt',
        'text',
        'title',
        'class',
        'imgclass',
        'linkclass',
        'caption',
        'link',
        'target',
        'popup',
        'rel'
    ),
    'html' => function($tag) {

        $url     = $tag->attr('image');
        $alt     = $tag->attr('alt');
        $title   = $tag->attr('title');
        $link    = $tag->attr('link');
        $caption = $tag->attr('caption');
        $file    = $tag->file($url);
        $autoLightbox = empty($alt) ? true : false;
        $thumb   = false;
        $url_2x   = false;

        // use the file url if available and otherwise the given url
        $url = $file ? $file->url() : url($url);

        // alt is just an alternative for text
        if($text = $tag->attr('text')) $alt = $text;

        // try to get the title from the image object and use it as alt text
        if($file) {

            if(empty($alt) and $file->alt() != '') {
                $alt = $file->alt();
            }

            if(empty($title) and $file->title() != '') {
                $title = $file->title();
            }


            // force a thumbnail
            $maxImageSize = c::get('maxImageSize', '800');

            if( $tag->attr('width') and $tag->attr('height') ) {

                // $thumb = thumb($file, array('width' => $tag->attr('width'), 'height' => $tag->attr('height'), 'crop' => true));
                $thumb = $file->crop($tag->attr('width'), $tag->attr('height'));
                $thumb_2x = $file->crop($tag->attr('width') *2, $tag->attr('height') *2);

                $url = $thumb->url();
                $url_2x = $thumb_2x->url();

            } else if(!empty($tag->attr('width')) or !empty($tag->attr('height'))){

                // $thumb = thumb($file, array('width' => $tag->attr('width'), 'height' => $tag->attr('height')));
                $thumb = $file->resize($tag->attr('width'), $tag->attr('height'));
                $thumb_2x = $file->resize($tag->attr('width') *2, $tag->attr('height') *2);

                $url = $thumb->url();
                $url_2x = $thumb_2x->url();

            } else if($file->width() > $maxImageSize or $file->height() > $maxImageSize){

                $fit_to = $file->isLandscape() ? 'width' : 'height';

                $thumb = thumb($file, array($fit_to => $maxImageSize));
                $thumb_2x = thumb($file, array($fit_to => $maxImageSize *2));

                $url = $thumb->url();
                $url_2x = $thumb_2x->url();

            }

        }

        if(empty($alt)) $alt = pathinfo($url, PATHINFO_FILENAME);

        // link builder
        $_link = function($image) use($tag, $url, $link, $file, $thumb) {

            // if(empty($link)) return $image;

            if(empty($link) and $file){
                $link = $url;
            } else {
                return $image;
            };


            // build the href for the link
            if($link == 'self') {
                $href = $url;
            } else if($file and $link == $file->filename()) {
                $href = $file->url();
            } else if($tag->file($link)) {
                $href = $tag->file($link)->url();
            } else {
                $href = $link;
            }

            if($link == $url and $file){
                // return html::a(url($href), $image, array(
                return html::a($file->url(), $image, array(
                    'data-size' => $file->width() . "x" . $file->height(),
                    'data-share-src' => url($file->url()),
                    'rel'    => $tag->attr('rel'),
                    'class'  => $tag->attr('linkclass'),
                    'title'  => $tag->attr('title'),
                    'target' => $tag->target()
                ));
            } else {
                return html::a(url($href), $image, array(
                    'rel'    => $tag->attr('rel'),
                    'class'  => $tag->attr('linkclass'),
                    'title'  => $tag->attr('title'),
                    'target' => $tag->target()
                ));
            }

        };

        // image builder
        $_image = function($class) use($tag, $url, $alt, $title, $url_2x) {
            if($url_2x){
                return html::img($url, array(
                    'width'  => $tag->attr('width'),
                    'height' => $tag->attr('height'),
                    'class'  => $class,
                    'title'  => $title,
                    'alt'    => $alt,
                    'srcset' => $url_2x . ' 2x'
                ));
            } else {
                return html::img($url, array(
                    'width'  => $tag->attr('width'),
                    'height' => $tag->attr('height'),
                    'class'  => $class,
                    'title'  => $title,
                    'alt'    => $alt
                ));
            }
        };

        if(kirby()->option('kirbytext.image.figure') or !empty($caption)) {
            $image  = $_link($_image($tag->attr('imgclass')));
            $figure = new Brick('figure');
            $figure->addClass($tag->attr('class'));
            if($file) {
                $figure->addClass('zoom');
            }

            $figure->append($image);
            if(!empty($caption)) {
                $figure->append('<figcaption>' . html($caption) . '</figcaption>');
            }

            return $figure;
        } else {
            $class = trim($tag->attr('class') . ' ' . $tag->attr('imgclass'));
            return $_link($_image($class));
        }

    }

);