Picture of lower quality

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.

Potato-quality answer

The standard term in contemporary internet vernacular to describe low-quality images is:

Potato

It sounds absurd, and it is, intentionally (we live in a very sarcastic age), but this is the standard term on the web.

No, I'm not kidding

Per the Internet-slang dictionary:

potato quality

adjective

of a photo, poor quality. From a photo looking like it was taken with a potato.

Sorry for the potato quality photos of the birthday party.
See more words with the same meaning: bad, poor, sucks, common, generally displeasing.

From the internet-trend documenting site KnowYourMeme's entry on the meme potato:

"Recorded with a Potato” is a phrase commonly found in YouTube comments criticizing the resolution of a video that appears heavily pixelated or blurry.

From Reddit, the gazed-navel of the internet, on the same subject:

It's ... a joke that their camera is so low tech, it's equivalent to a non technological item like a potato.

And

See also the B3ta website, where "potatoshop" or "potatochop" was often used for low quality image manipulations, often followed by someone exclaiming "Seamless!".

This was happening in at least 2003.

Google searches show it happening in like 2002.

And

The story behind it is that potatoes don't take very good photos.

Lifestyles of the rich and shameless

The usage is so widespread that this high-quality image of a potato^🥔:

was bought by an Silicon-Valley internet-tech millionaire for over a million dollars. Ironically.

Welcome to the internet.

^🥔_{Credit to @Lawrence for finding this article}

The pictures on xyz.com are of low resolution.

Or in general, if they are not of low resolution, low quality works fine.