Materials old and new
I’ve been preparing illustrations for publication since the early eighties. Back in the day, materials used were typically pencil and ink on paper.
Today, when producing for the web I frequently start with pencil, paper, and ink even if the final image will be created in Adobe Illustrator and Photoshop; a thumbnail sketch can act as a springboard to a full composition.
Adding digital and photographic elements to a traditional drawing, adds immediate interest and depth:
Social media
Web publishing is more flexible than print media, it’s squishier. In print, a trade size paperback has specific dimensions that remain constant (135mm x 216mm), while a web page may be viewed at a width of 390 pixels, and 1280 pixels (and more) causing images and text to move around and, often, resize. Throw in ever-varying screen resolutions and default cropping used in social media previews, and soon image making for web devices grows to be quite a task when images contain critical information.

If it has been a while since I’ve published to Facebook, I’ll have a look at their publishing guidelines to see current image recommendations. There I read that Facebook suggests using these widths for “Regular photos” (in posts): 720px, 960px or 2048px, and for “Cover photos” (banner images), a width and height of 851px by 315px. Repeat for Twitter, and other platforms. Testing on the site, or app, is crucial if there is information in the image that must be visible.
Storytelling
Usually, not always, an illustration assists the text. A little research and discovery can point the way forward. Here is an illustration that would agree with and reinforce the general proposition made by the text on a Contact or similar page:

The screen-shot below shows an illustration that supports the story in a less explicit manor, permitting the reader to fill in with their own imagination (courtesy of Inner Life Directions):

Finally
The main thing is to work with the methods and materials that you’re comfortable with, do your research, experiment when you can, and have fun!
what’s great is that however you get there it works!