Images will shrink to best fit the desired size, the image will not be cropped, but the images will not be exactly 800×600.
The links that are provided at check out and email have and expiration of 24h, but if the user has a customer account they can download the images anytime.
If you want to implement your own link see line 485 image-store/_inc/cart.php and you can use the hook ims_image_tag_meta to add your link bellow each image.