Images are often created for the web using image editing software. There are a lot of options for which software to choose. A Google or YouTube search for “top image editing software” should be a good starting point for research.
Here is a list of the top image editing software for 2021 according to pcmag.
There is also a very good free image editor called GIMP.
Obviously there would be different options if you are using a MAC or other type of computer.
Once you have chosen which software to use there are lots of places to find tutorials on how to use it. For example, go to YouTube and search for “photoshop tutorial for beginners”.
I’m sure members of the community will have lots of ideas about which is their favorite image editing software. I currently use Photoshop.