Short answer: Big! At least 2048×1536 if possible.
Slightly longer answer: Big. If you upload your images at a resolution bigger than the devices then My Tours will resize these for optimal display on the users device. We generate several sizes for different devices to optimise the download size and quality. Also note that our standard image viewer works best with images in a 4:3 ratio and portrait images are scaled (not cropped) to fit the image viewer. The user can of course use the zoom feature to view the image full size.