The "Using the <PICTURE> tag syntax" option doesn't work; next-gen images won't show up

The "Using the <PICTURE> tag syntax" option replaces the img HTML tags with picture tags so that ShortPixel can deliver WebP and/or AVIF files correctly. It is one of the two options you can use to deliver next generation images on your website. You can find it at Settings > ShortPixel > Advanced > Next Generation Images.

There are a few possible explanations for why this delivery method does not work:

• The WebP or AVIF version was not generated. Check out this article: I optimized already all the images but then I realized that I didn't select the option to generate next-gen images, what can I do?
• The images are served via JavaScript mechanisms.
• A cache solution is serving old pages. It can be your browser, server, CDN or a cache plugin. Try clearing them all: How to clear WordPress cache
• You have used a page builder for page creation. Our feature may not work well with pages created with some of these tools.
• The images are background images. If your images are loaded via CSS with the background-image property, ShortPixel Image Optimizer cannot serve them as WebP or AVIF using this method.
• The lazy-loading solution you are using conflicts with SPIO. Try disabling all lazy loading features on your website to see if this fixes the problem. If it does, we recommend that you use a different lazy-loading solution.
• You have a Windows Plesk server. You need to add a MIME type for .webp (image/webp .webp) or .avif (image/avif .avif) extensions on your server.

If none of these apply to your case, you should try an alternative solution for next-gen images delivery:

• Use the second next generation image delivery option built into SPIO (Without altering the page code (via .htaccess)): WebP instructions / AVIF instructions
• If you have an NGINX server, you can configure NGINX to transparently serve next-gen files when supported: WebP instructions / AVIF instructions
• Replace ShortPixel Image Optimizer with ShortPixel Adaptive Images.
• Use another plugin to deliver the WebP/AVIF files created by ShortPixel, such as Cache Enabler or WP Rocket.
• If you use LiteSpeed Cache, you can use it to deliver the WebP files generated by ShortPixel.