With 0.18 update the nsfw patch is missing yet, so in the meantime I found this userscript and modified it to work with NSFW.

P.S. I know this is not the best community for this, but nothing else fits.

Use your userscript manager of choice to install it. Violentmonkey, Tampermonkey

// ==UserScript==
// @name        Lemmy Image Expand (modified)
// @namespace   Violentmonkey Scripts
// @match       *://*/*
// @grant       none
// @version     1.0.1
// @author      SlyFabi
// @description Auto expands all image posts including NSFW ones.
// @license     MIT
// ==/UserScript==

(function() {
    'use strict';
    // Thanks to CodingAndCoffee for isLemmy
    let isLemmy;
    try {
        isLemmy = document.head.querySelector("[name~=Description][content]").content === "Lemmy";
    } catch (_er) {
        isLemmy = false;
    }

    if(isLemmy) {
      // https://stackoverflow.com/questions/18177174/how-to-limit-handling-of-event-to-once-per-x-seconds-with-jquery-javascript
      function throttle(func, interval) {
        var lastCall = 0;
        var nextCall = -1;
        return function() {
            var now = Date.now();
            clearTimeout(nextCall);
            if (lastCall + interval < now) {
                lastCall = now;
                func.apply(this, arguments);
            } else {
              nextCall = setTimeout(function() {
                lastCall = Date.now();
                func.apply(this, arguments);
              }, interval);
            }
        };
      }

      const targetNode = document.getElementById('app');
      const config = { attributes: false, childList: true, subtree: true };
      let observer = null;
      const callback = throttle(function(mutationsList) {
        if(observer != null) {
          observer.disconnect();
        }

        setTimeout(function() {
          let postList = [];
          document.querySelectorAll('.post-listing a.text-body svg.icon use').forEach(function(postIcon) {
            const imgPreview = postIcon.parentElement.parentElement;
            if(postIcon.getAttribute('xlink:href') !== "/static/assets/symbols.svg#icon-image") {
              return;
            }
            postList.push(imgPreview);
          });

          let uniqueList = postList.reduce((unique, o) => {
            if(!unique.some(obj => obj.getAttribute('href') === o.getAttribute('href'))) {
              unique.push(o);
            }
            return unique;
          },[]);

          uniqueList.forEach(function(imgPreview) {
            const postListing = imgPreview.closest(".post-listing");
            const isExpanded = postListing.querySelector('.img-expanded') != null;
              if(!isExpanded) {
              imgPreview.click();
            }
            //console.log('UElement: ' + imgPreview + ' Exp: ' + isExpanded);
          });
          /*postList.forEach(function(imgPreview) {
            imgPreview.style.pointerEvents = 'none';
            //console.log('Element: ' + imgPreview);
          });*/

          setTimeout(function() {
            observer = new MutationObserver(callback);
            observer.observe(targetNode, config);
          }, 500);
        }, 500);
      }, 1000);

      setTimeout(function() {
        callback([]);
      }, 500);
    }
})();