I see, let me, does the slide show have transitions that move the images during transitions, or is is just a solid image appears and then eventually switches to another one without any transition (fade, slide, etc.).
If this is the case, you should be able to find some mp4 or other video codec that compresses it well.
But if you have transitions between the images, then any video format you use is likely to result in a significantly bigger file.
In that case (or if you just want a more flexible and efficient and less burdensome for the users approach) your solution would be to find an html/javascript script designed to run the slideshow that you can upload with your images.
In this case, the html/javascript will be serving the same function as irfanview does on your local pc.