Our system optimizes images with 4:3 ratio.
For items, we suggest to use horizontal images. If the image you're going to use is horizontal, 768×512 or 1536×1024 would be ideal. For ipads, please use 1536×960 pixel images.
For backgrounds, vertical images work the best. If the image is vertical, please use 768×1024 or 1536×2048. The same size is valid for items as well but please be aware that the system will focus on the photo automatically to display it as a horizontal image which means it would not be displayed fully.
To reduce the size of your images, you can use this link https://bulkresizephotos.com/