Rickey is correct. By default, if use pictures in a Shortlist that are larger than 280 x 210 pixels width/height, it will scale them down to fit into the Shortlist popup nicely. That's why we recommend ideally using pictures that size and shape.
(You don't need to resize your pictures down to that size because the Shortlist does that for you so they fit, but if you use very large pictures it can slow down the initial load time of your Shortlist because those pictures are also displayed in the tabs).
If you want the Shortlist to display larger images in the popup or details panel, you would need to customize the code. I found a few examples of this:
http://maps.nashville.gov/LID_Sites/
http://mediamaps.esri.com/geography-of-horror/
http://www.iau-idf.fr/urbaviz/shortlist/
http://www.nps.gov/gis/storymaps/shortlist/v2/?appid=963b3a1ab77e4e4bb20660f7dab8b3cd
(in this example clicking on the picture in the popup display a fullsize version of it).
We want to improve how Shortlist handles pictures because everyone likes great pictures and by default they are quite small in Shortlist. So in a future release we would like to support larger image display.