A small set of image sizes helps editors avoid uploading huge files for thumbnails. Begin by looking at the spots where images appear.
Measure the approximate width and aspect ratio of the article cover, small card and product tile. Save a reference image for each shape. Crop around the subject before exporting, then compare a smaller copy with the original at its actual display size.
When an image is an illustration or logo, check whether the format preserves clean edges and transparency. For a photo, balance fine detail with file size. Put the size guide where an editor can find it while uploading.