Modern Magazine registers its own crop sizes. WordPress generates these when you upload an image, so upload at or above the sizes below and let WordPress crop down.
Registered sizes
modern_magazine_ThumbnailImage— 840×560 — used for slider images, homepage section thumbnails and blog listing images.
If your images look wrong after switching themes
WordPress only generates crops at upload time. Images uploaded while a different theme was active will not have Modern Magazine’s sizes, so they fall back to full size and can look oversized or soft.
Fix this by regenerating thumbnails — the Regenerate Thumbnails plugin does it in bulk. Run it once after activating the theme.
Featured images
Posts without a featured image degrade gracefully — the layout closes up rather than leaving a gap.