Pippa - Recommended Image Sizes
FEATURED IMAGE SIZE
For your featured images, images should be at least 1000x750px, but no larger than 2400px. That size will ensure your images display correctly in all areas of the theme, including category pages, blog page, the image slider on the homepage, etc.
POST IMAGE SIZE
The width of the blog post column is 800px. For the clearest possible display, we recommend uploading images roughly twice that size (1600px is great).
If page speed is a concern, you can upload images right at 800px, 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.