Has nothing to do with wppizza. WPPizza does not set image sizes like that.
it only ever gets the thumbnail depending on what’s set as thumbnail sizes
and generates and outputs something like this:

<img width="64" height="64" src="http://[somedomain]/wp-content/uploads/2022/08/pizza-64.png" class="wppizza-article-image-thumb wp-post-image" alt="" title="Special Pizza" />

your theme – or some plugin – generates this:

<img loading="lazy" decoding="async" width="768" height="768" 
src="https://[somedomain]/wp-content/uploads/2024/10/pizza_pomodori.png" 
class="wppizza-article-image-thumb wp-post-image" 
alt="" 
title="Pizza POMODORI" 
srcset="https://[somedomain]/wp-content/uploads/2024/10/pizza_pomodori.png 300w, https:/[somedomain]e/wp-content/uploads/2024/10/pizza_pomodori-150x150.png 150w, https://[somedomain]/wp-content/uploads/2024/10/pizza_pomodori.png 768w, https://[somedomain]/wp-content/uploads/2024/10/pizza_pomodori.png 1024w, https://[somedomain]/wp-content/uploads/2024/10/pizza_pomodori.png 800w" 
sizes="auto, (max-width: 768px) 100vw, 768px">

you will also find that where your images look the right size the last declaration is

sizes="(max-width: 768px) 100vw, 768px"
instead of
sizes="auto, (max-width: 768px) 100vw, 768px"

i.e whatever generates the “auto” is your culprit. Only you will know whether that’s a theme or plugin. I can guarantee you though it’s not wppizza

Just out of curiosity, I had a dig around. So below some comments which may (or may not) be helpful
Not exhaustive research though. Maybe it helps someone somewhere.

>If I upload the pictures directly it seems to work.
Yup. If you upload pics the regular way with wordpress, it generates 4 distinct separate images scaled and/or cropped.
exmage-wp-image-links (other/similar plugins may do the same) simply uses the image you added which then gets resized via css and image attributes. No actual resizing/cropping is taking place.
Since WP 6.7 WP adds the ‘auto’ attribute – if it’s missing and in certain circumstances – to the image sizes attribute.
So if the actual (thumbnail) image is meant to be square – but the actual image is not – it will simply be distorted to make it square.

As it turns out, you can actually turn this off with the filter below, but this might have implications elsewhere.
I did not want disappear all the way down that rabbit hole – so if you do that , make sure to check all your images / pages .
Perhaps it works or perhaps it’s just a starting point . All depends on your particular site setup and many other factors. Suffice to say, it’s out of my (wppizza) hands and evidently something that WP introduced to the detriment of these kind of plugins…

//remove auto from image sizes attribute (if set to lazy) 
add_filter('wp_img_tag_add_auto_sizes', 'myprefix_img_tag_remove_auto_sizes');
function myprefix_img_tag_remove_auto_sizes(){
return false;
}