Product images - Ideally you should be using an image no smaller than 1000px tall or wide to have good quality product pictures. The bigger the images the better the resolution of those will be and on product pages the better the image zoom detail will be given
Slideshow - When shown at full screen width use: 1920px wide x 800 - 1000px tall
The height of the slideshow depends entirely on the height of the images that you use. It is imperative to keep all the same size and to make sure that you use landscape oriented images.
Image Galleries - The size of images used depends on the setup that you are using, there are numerous options within the theme, here are some suggested sizes.
If you are showing images in identical blocks in a grid 3 or 4 per row we recommend working with 600px wide x any height ( keep each image the same size ).
If within an image gallery you are showing just 1 image at full screen width we recommend that you use 1920px wide to ensure you have a high quality image. The height of this section will depend on the height of the image that you use.
If you are looking for a more advanced configuration as shown on our Secret Sale demo:
For this we show 4 images within 1 section.
Image 1 = 33% width ( Image used is: 438px wide x 583px tall )
Image 2 = 33% width ( Image used is: 438px wide x 291px tall )
Image 3 = 33% width ( Image used is: 438px wide x 291px tall )
Image 4 = 66% width ( Image used is 904px wide x 272px tall )
Experimenting is the best way to choose the layout.
If you are looking for a quick method to creating images for your shop and want to test some resolutions we recommend using https://unsplash.it/