What sized images should I use for my slideshows (for desktop)? I'm unsure, but at the moment I am using 1920x1080 images (16:9 aspect ratio) with text placed on the images. I then use negative margins to get the text positioned where I want it. Is this the right practice? Or should I be using smaller images? I have to think of those people with Retina monitors etc. If the images are too small then they will appear pixilated on decent monitors.

What ever you think looks best as far as dimensional size. I would be more worried about physical size.