When you add images to ProSelect, by default it creates screen-sized copies of those images and loads them into the Album file.

If you are preparing your images on a computer with a smaller screen size (in pixels) than the computer that you will be displaying the images on, then you should go to ProSelect's Preferences (under the Edit menu)  -> Importing Images -> Settings -> Adding Images --> Resize to:

Then select the size of the large screen (or the closest next larger size). Sizes up to 2400x1600 are available.

 
If your original images are smaller than the size of the screen then ProSelect will NOT scale them up during the slideshow but WILL scale them up when in Show Images view if you have "Expand Small Images" ticked in the View Menu.

You can also REDUCE the size of you Slideshow images by setting Image Size (as a percentage of the screen size) in the Preferences (Slideshow Tab). This is useful for adding a border around the images.

* (In version 3.x: Preferences  -> General -> Adding Images --> Resize to: with sizes up to 1920x1200 available)