The Presto template has the ability to resize and cache images used in your gallery. When an image is added to a gallery it is automatically resized based on the current resize settings.
Images are processed and saved to the media/presto-gallery/cache/images folder. They are renamed and given an md5 hash which provides a unique file name for that instance of the resized image.
The version of the image used in your gallery layout is controlled in the template itself. The resized images displayed have been matched to suit the maximum width of images shown at various screen sizes according to the specific details of that layout. That is to say in the 3 column grid layout small images are still shown on wide screens whereas images for slideshows etc show a larger version of the image.
Some layout templates like the Fullscreen layout display the original source image.
The original image is shown in the lightbox display.