Website developers are not responsible for creating assets.

The main difference between web design and web development is that while web designers make visual assets that add aesthetic value to the site (and enhance user experience), web developers are responsible for adding those functionalities.
The developer will translate the designer's mockups, wireframes, and other assets into code using JavaScript , CSS, and HTML. So the creation of visual assets is the responsibility of the website designer, whereas using code to make those pop up on the website is the job of the website developer.