How to view images at actual size in slideshow (by default)?

Is there a way to view images at 100% of their size in slideshow by default?

When I select a number of images and play them in a slideshow I wish to avoid pressing option key for each of them to show them at actual size (the real-world scenario would be having a bunch of taller images, such as mock-ups of web pages, that are then squeezed on the screen and practically indiscernible).