The artwork files are stored on IPFS and a unique hash to the metadata (including a hash URL to the downloadable image on IPFS) is generated each time and stored in the metadata (which can be viewed on Etherscan).
If you look on Etherscan at the transaction for the original tokenization of the file, click "Click to see more", then "View input as UFT-8" you will see the unique URL to the metadata which includes the image hash URL.
Example for this artwork:
At the bottom of the history, you will see the original transaction for when the token was made. If you click "View tx" it will take you to etherscan.
Original tx link for artwork above: https://etherscan.io/tx/0xe7c8bc93adaa583d4f7f8bff0d33707820d3dce4ec19fb86cbd328a609a5c96f
Once on Etherscan:
Click the "Click to see more" link at the bottom left
Click "Decode input data" button beneath the gray box
Copy and paste the url and open it in your browser. In this case, that's https://ipfs.pixura.io/ipfs/Qmeh8RYEvqi9eib83sqDKJAj7emby1K1A4UUWiQdk9LHzn/metadata.json
To view and download the image copy the image URL and paste in your browser:
To download the file, just right-click and "save as".