When uploading blog images, remember that these images should be suited for sharing across social media channels. Generally, blog images nest well on a website at 3:2 aspect ratio or 1200px by 630px. Blog images in landscape format are best sized to 1200px by 900px and featured blog images in portrait format are best sized to 900px by 1200px.
High-quality images with small file sizes should be uploaded for blog images. For blog images, the largest recommended file size is 150KB. The smaller the file size, the better the speed of the blog. For the CMS to adapt blog images to different sizes, they should be 72ppi with the focus area centered.
Source: https://tiny-img.com/blog/best-image-size-for-website/