Learn English – Word for a single picture which can be interpreted in two different ways

Based on the wikipedia entry that looks very much like the relevant part of the document you linked to, it is technically a conjoined twin and colloquially a navel. There is also some precedence for calling it a baby orange.

The mutation caused the orange to develop a second fruit at its base, opposite the stem, as a conjoined twin in a set of smaller segments embedded within the peel of the primary orange. - wikipedia

A Presbyterian missionary came upon it in the mid-1800s. It intrigued him that not only did the orange have a bellybutton and baby orange inside — it was sweet, and had no seeds. - Who Put The Navel In Navel Oranges?, npr.org

However, when saying that you've eaten that portion of the fruit, it's probably more socially acceptable to say that you've eaten the baby orange than to say that you've eaten an orange's navel.

You're referring to compression artifacts. The pictures can be described as degraded.

degraded adjective 1.1 Reduced in quality; inferior. ‘It is necessary to design the antenna properly or system performance will be degraded.’ - ODO

Here are a couple of examples of the term in use:

• A quality setting of 100 does not degrade an image at all: False - JPEG Myths and Facts, ThoughtCo

• If it's saved and closed again, compression removes yet more quality, and so on as the image gradually degrades. - JPEG Image Compression Degradation, patricktaylor.com

Your sample sentence would be:

• Pictures on site xyz.com are degraded.