Featured image

I’m not sure if the featured images are scaling right. I’m not trying to make featured posts. Just trying to display images for the posts on the home page. I was having trouble with it last week and they just readjusted themselves when I checked my site the next day. It is happening again when I try to add new featured images. Do I have to rescale the featured images to a certain image size? The preview doesn’t look like what will be posted. Confused. 🙂 Do I do it in jetpack???