Audrey - Recommended Image Sizes
Featured Image Size
For your featured images, images must be at least 800x1000px, but no larger than 2400px. That 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 780px. 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 780px, 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.
Genesis Responsive Slider Size
If using the Genesis Responsive Slider option in Front Page 1, your featured images for the slider should be no smaller than 2000x1000px.