You’re very welcome to use images that are larger than noted below, because whenever a photo is uploaded to your blog, it will be automatically generated/copied to many image sizes to be sure the page loading speed is optimized. Please note that using very large images (such as the orignial from camera) where it is not necessary can affect your site’s loading speed and may even cause image uploading errors.
Full width post image: Image width at least 1240px.
Standard post image w/ sidebar: Image width at least 775px.
Thumbnail images: Smaller images such as grid layout, list layout, related posts, etc., will take your post’s featured image and automatically crop it so that the image fits nicely within the smaller container.