Images that are royalty-free are distributed on websites that aim to help their owners assemble aesthetically pleasing pages without the need to hire artists who can create custom illustrations. There are aesthetic elements that can be purchased on websites so that web designers can create websites that appear far more professional than what those designers might be capable of creating on their own. However, there are many options for relatively simplistic images that can be made use of without having to worry about either financial costs or the need to credit the source that provided the image. Depending on the designer's artistic talent, these images can make smaller websites highly pleasing to look at and navigate in their own right.
Programs and applications that are open-source are those that have been released by their creators alongside the source code that was used to compile them. Since it is not feasible to reverse-engineer a software program, programmers who willfully share the uncompiled source code behind their projects do other programmers a large favor because other programmers can study and borrow the code and make their own adjustments and expansions to the programs.
The distinction between royalty-free media and open-source software is apparent on a website named unDraw, which claims to offer "open-source illustrations." The "Illustrations" page contains many hundreds of technically simplistic yet aesthetically striking drawings that can be freely downloaded as SVG files. The website's "License" page explains how users can include them in their own projects without the need to credit the resource. This is very much how these images can be claimed to have been provided royalty-free if an audience member recognizes that an image on a web page may also be on a completely unrelated website.
On the other hand, these images cannot be accurately described as open-source because they are not themselves programs that offer the tools necessary to create, modify, and export other images. The confusion from unDraw appears to have stemmed from how the image files are technically SVG files made out of syntax that can be manually adjusted "at the source." For more information click here https://undraw.co/illustrations.