Welcome!
For photos, in a post you can click right under the text box, I think it says "select them," and upload from your computer; or you can drag and drop (so I'm told; my only internet is a Kindle so I can't do it that way), or you can c&p the link from it elsewhere on the internet. So say you have posted a photo on Facebook, you can then open it full size in your browser (not the post, just the photo part), then copy the URL and put that in your Steemit post text box. That copy-paste is how you can put a photo in comments, as well. Like so: