How to make the displayed article thumbnail initially small?

Question

Joomla

Good evening.
There is a question.
In my template, products (articles) are displayed, they logically display thumbnails and headings. The only problem is that the thumbnails are initially displayed ORIGINAL, and this is bad, because they take a long time to load.

<a href="index.php?option=com_content&view=article&id=<?php echo $r->id; ?>"><img class="proverka" src="<?php echo $r->gallery_img1; ?>" alt="<?php echo $r->title; ?>" title="<?php echo strip_tags($r->introtext); ?>" /></a>

This code is responsible for displaying the image.
What needs to be written so that the picture is initially displayed not full, but for example a small thumbnail?

Answer 1

Well, there must be a field for a thumbnail somewhere. Can you at least tell me what you use: materials or K2 or something else?