When you define image sizes in WordPress the images are actually cropped when they are uploaded so if you are switching to this theme from another one and want to use the image sizes defined in Zero you’ll want to Regenerate your thumbnails.
We recommend the following plugin: Force Regenerate Thumbnails
This plugin will not only crop your images for use with the theme, but it will also delete any old cropped images that are no longer needed saving you precious server space – yay!
Default Theme Image Sizes
Both entries and posts have a default cropping value of 940px by 480px.