Yes, it's always been considered poor internet etiquette to post someone else's photo embedded without permission.
On the other hand, a link to the photo or site is always OK. The reason for the difference is in that case, you get the copyright info, the photographer gets credit, and it's presented in a way that the photog or webmaster chose. example
http://www.pbase.com/roberthouse/image/45786503Embedding that same photo would put the bare photo on someone else's page, with none of the information, just the photo itself, as though it's the property of that person. And, to top it off, it uses their server and bandwidth to serve up the photo every time someone looks at on 'your' page. If you want to do that, you should ask the photographer and offer to credit the photo to them, and/or add a link to the page where they have it shown.
On my pbase site, it tells me if there's a direct link somewhere to a photo. Some years ago I shut off direct linking, so for example if you tried to embed that above photo, it wouldn't work. Many providers also won't allow direct linking, and you get that red 'x'. I keep a couple of galleries with direct linking enabled, so I can put photos there and use them in forums like this. But you'd be surprised how many people will just, for example, make up a Yosemite page, and just direct-link to other peoples' photos to fill it up, with no credit or anything.
One might argue that they can just download your photo and use it, and that's right, but at least you're not stealing their bandwidth on top of their photo. And, by doing that, more people might think twice about whehter it's really OK to be posting that photo. In cases like that, there is recourse through the person's ISP or photo or website host, and they do take copyright violations pretty seriously
Some situations are different, like public domain photos the webcams etc., and generally they don't care; they're (webcams) just robotic photos anyway, so there's no real issue of stealing someone's hard-earned photo.
Personally, I figure if you post a photo on the web, you would be naive to think no one will use it for something. With my own photos, I don't care as long as it's not direct-linked without asking, or for any commercial use or profit. Someone wanting it for wallpaper, or a card or just a photo to look at, I don't care, and even if I did, there's not much one could do about it...that's the internet.
While we're on photos, one thing I'd like to mention is that most people have a monitor that might view only 1280 pixels wide, or maybe 1024, and part of that is used up by frames and menus; so there's no real point in posting a huge photo, except to make people scroll to see it. 800 or so pixels wide is a nice size that is large and viewable, and fits in most everyone's browser. It's really counter productive, most of the time, to post a photo that people have to scroll sideways to try to see (I usually just don't bother).
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo