1) Create a "Paper Size" and call it "Digital JPEG" or something similar and set a wholesale price of $1. Create more items if you plan to offer multiple image resolutions.
2) Create a "Paper Type" and call it "Delivered via Email" or words to that effect and set a wholesale price of $0. You could even create an additional "Delivered via Disk" option if you like.
3) Create a "Shipping Option" item and call it "No shipping required (download only" or whatever you like.

With these items set in your wholesale settings you can now create/edit your feature sets and pricing models that will be applied to your Galleries.

5) Click on the Feature Sets button
6) Edit a Feature Set (choose Default Feature Set if you have not created any yourself). Move the new "Digital JPEG" item by ticking the box and clicking the IN button to move it from Available to Active.
7) Do the same for the Paper Type items, move them from Available to Active
8) At the bottom of the page, select the product combinations you want to offer for your digital download (see screenshot attached)
9) Click on the Pricing Model button
10) Edit a Pricing Model (choose Default Pricing Model if you have not created any yourself). Add the retail pricing for the "Digital JPEG" Paper Sizes you wish to offer and set the "Delivered via Email" option to $0 (or 0%) - just dont leave them blank.

11) The last step is to ensure that the Feature Sets and Pricing Models are applied to the Gallery. Click on the Photos tab, and then click "edit" for the Gallery you wish to have the digital downloads available for. At the bottom of the pop-up window, select the Feature Set and Pricing Model that you added you have just edited. You can then apply these same Feature Sets and Pricing Models to any other Gallery that you want to make these options available in.