If you are using the slideshow in ProSelect v2013r1 and newer and the images look blurry, then likely this is your preferences setup.

Please see the article "Images look pixelated or soft on the screen or projector" - the section on "OpenGL Upscaling (effects slideshow in ProSelect 2013r1 and newer)" for what you can do.
 

Note:

If your images imported into ProSelect are not sharp, (and they look sharper in another program such as Photoshop), then this will be caused by a different reason:

1. The article above will also assist with this.
2. Possibility that you have imported images that are too small a size and they are being scaled up to fit a larger screen space. See the section "How Far Can You Zoom?" in the article Zooming in for more detail for how to control this in ProSelect.  Then after adjusting this setting, you need to Re-Load All Images (see "Reloading the Images" section in the article Replacing All Imported Images).