You should be aware that many vendors currently provide computer-simulated images that are based on computer scans of the diamond. These simulated images can represent the diamond relatively well but please note that it is possible that the images can contain scanning errors and this is why the images look different to actual photographs of hearts and arrows and ASET images.
The best way to tell a computer-simulated image is to familiarise yourself with real light performance and optical symmetry images. You can read the light performance tutorials or the hearts and arrows tutorials to see examples. Another way to tell is to compare the arrows and the ASET images to see if they are too much of a perfect match for each other as if it could only be replicated with a computer.
If you think having an image generated from a computer scanning of the real diamond is bad, then you should be prepared that there are some vendors out there that are even worse. I have seen computer-generated images that are generated without scanning the actual diamond where the vendor will simply input the proportions on the GIA certificate and generate images that have seemingly perfect symmetry.
The takeaway here is to always question when you have any doubts and to always purchase from a trusted vendor with lots of good reviews.