Featured Images; disabling image downloads?

Hello, thanks for your help. I read up a bit on Featured Images here your site. I still don't get exactly what is the overall function of the "featured image." Since it's extra work I'd like to know why I should (or shouldn't) choose one for each page and post. (1) Is it possible to have a default of whatever photo is embedded in the content to be the featured image? (2) Is there something wrong with NOT choosing a featured image? (3) Can the size of the cropped featured image as it appears in the abbreviated blog-snippet of a post be adjusted? (4) Is there an SEO advantage to setting featured images on things like Pages that are not showing up in blog-scroll summary? Regarding possibly using the Slider: (5) Can the Slider use JUST the Featured Images and not include words from the posts? Also: (e) Are there any options within the theme for disabling download of images? (and watermarking?) (f) Also, for all of the above, do these all function the same in Pro or does Pro have any other options with regards to these items? Thank you.