Ivy - Recommended Image Sizes
Featured Image Size
For your featured images, images must be no smaller than 750x1000px. Larger is fine. 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 750px. 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 750px, 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 900px and images should be at least that size.
Carousel Slider Image Size
Images in the carousel slider display at a set height with variable width. For the best display, images in the slider should be sized at 900px (height) with any desired width.