With Sytist, you can sell prints and downloads. But there is also an option with your print products to include a download of the photo for a price you set.
To set this up, go to Photo Products -> Product Base and click Options next to the product you want to add this option to and click Create new option.
In the option edit, give a name for the option and in "Option Type" select "Download". Fill out the amount to charge and the max. pixels of the download. If you want them to download the original size download, then enter in 0. You can also give it a description and there is an option for "Do not allow download until I upload a replacement file or manually approve it".
Now when the customer is purchasing this product, they have a checkbox to check for the download product.