What’s a featured image? If you’re not sure, read this first.
In addition to featured images, you’ll probably want to place images within the body of your content.
Upload the largest version you have (max 16 MB) and let WordPress resize the image dimensions for the space where it’s displayed.
This gives you flexibility to use a photo different ways, e.g., once as an image on a card, where it may show up small, and again as a featured image on a post or page, where it would display much larger.
Orientation and aspect ratio
You’ll be set 90% of the time if you pick images that are horizontal, at least 760 pixels wide and have a 3:2 aspect ratio.