Each slide of a slider is a box. It's your choice how to populate it. A background image for decoration, then text and image content as primary content. That animated part is only for the eye. Computers and search engines only see a series of boxes with their respective content, starting with the first slide and finishing with the last one.
Hahaha that's the most open question you can ask I coul write a book as an answer
First, PNG is not PNG. I mean there are a lot of different ways to produce PNGs and their specs are wide. PNG can be 8bits total (with a 256 color palette and 1 solid color of transparency), or 8 bits per channel with a 8 bit alpha channel (24+8 32 bits), can be non destructive, can be destructive...
8 bit PNG is like a GIF. 32 bit PNG is for when you need a 8 bit alpha channel (very good and refined transparency). If you image is just a complicated graphic with gradients or a photo, JPEG is the way to go. Adjust the compression from 45 (very compressed) to 75 (almost unnoticeable compression) and you'll get the best size you can have.
Read here when to use gif, jpeg, png, svg, and why: https://www.sitepoint.com/gif-png-jpg-which-one-to-use/
Also, my advice is to use Photoshop to generate jpegs and gifs, there is nothing better. My second advice is to always, always use a third party image optimizer. Among many I recommend ImageOptim https://imageoptim.com/mac
I also like very much the brand new Optimage, by Vlad Danilov. He seems to be an image engineer and knows a lot about loosless image compression.
You can make your own researchs too, the topic is endless and quite very important for what we do.