You can embed videos from different sources that allow it and the length of the video will be the same as the original one. Well, you can embed photos as well 🙂
I would not recommend downloading porn scenes from paysites and then uploading them to your site, because you will be hit with the DMCA notices like mad and if you do this on a big scale, then you might even end up having some legal issues.
It depends what kind of website you want to run, but some sponsors in some niches give you free content that you can use to promote them, so you should consider doing this.