Featured Images are images tied to an article that are picked up by social media and displayed alongside a summary of the article. Hence, a featured image is important because it can be the deciding factor if someone clicks on the link or not. Remember though that it needs to be dynamic as all devices are different. So any text should be centered, ideally there is no text at all. Sites may automatically crop featured images so whatever is in the middle and center may be all that's left.
We use Snappa to make featured images quick and easy.
You can use Photoshop which is actually preferred but this will take much longer per image and the quality difference is negligible.
See a few examples of featured images we made in Snappa:
Get Snappa, A graphics program, to make shiny nice looking featured images, banners, and backgrounds.