The famous and infamous blind.gif, a transparent square, maybe the most frequently used image in the early days of the web before 2000. It was used, to give an area on a website a certain size, mostly in table layouts. Due to the advanced possibilities of CSS, it is no longer widely used.
<a href="//commons.wikimedia.org/wiki/User:Jurii" title="User:Jurii">Jurii</a>
<span class="int-own-work" lang="en">Own work</span>