Harper - Recommended Image Sizes
Featured Image Size
For your featured images, images must be at least 800x1100px, but no larger than 2400px. This size will ensure your images display correctly in all areas of the theme (front page, blog page, category pages, etc).
Post Image Size
The width of the blog post column is 730px. For the clearest possible display, we recommend uploading images roughly twice that size (1500px is great).
If page speed is a concern, you can upload images right at 730px, but your images will be fuzzy on retina devices. Larger images don't really affect page speed all that much, unless you have an excessive number of images in one post.
If you're using the full width post layout, the post column is 950px and images should be at least that size.
Post Slider Homepage Option
If you're using the Post Slider Homepage Option, your featured images will need to be larger to accommodate the slider size.
Content Width Post Slider: 1100x1100px minimum featured image size
Full Width Post Slider: 2000x1100px minimum featured image size