The Bel Canto theme uses flexible image sizes for all the images used throughout the theme.
This means that all your images will appear in full aspect (height & width) and not be cropped in any manner by the theme.
This also means, if you like a very neat and orderly view or are using templates like the Lookbook, Portfolio or Category Archive, using Featured Images that are the same size will produce the best results.
Slider Images - Slider image sizes will be based on the slider settings you use for your slider plugin. Please see the plugin documentation for more details on settings.