Recommended images sizes
For the homepage full horizontal slideshow images we recommend to use 1920 x 1040px size images but please note that any fullscreen image will require a level of cropping in order to display as fullscreen on any screen size (for this reason we recommend to avoid using important elements of the image on the edges).
Because for the collection page we have 4 different options in theme customizer, there isn't a recommended ratio for the uploaded images here (please test with the available option what works best for you).
We also always recommend to upload light assets: optimized JPG images (up to 90% optimization) not exceeding 100-200kB.
For the fullwidth sections (like the slideshow or "cropped" product images) a level of cropping is required in order for the images to display as fullwidth on any screen size. Please use the recommended images sizes to minimize the cropping and also avoid using important image content (like text for example) on the edges.