I think we need to define the difference between a photo sharing site and a photo hosting (or sharing) site that allows transferring.
What you're referring to, emeraldsky, is a hosting site that allows transfers. There are many photo sharing sites that do not allow transfers or allow transfers only to members paying for that service.
You're right, if you try to transfer a photo to a post on C-D forums from a hosting site that does not allow transfers, you may be able to see the photo (in some cases) but all other members will see either nothing at all, the infamous "red x," or a message that the photo is unavailable.
The two free ones that come to my mind immediately which allow free transfers are photobucket and flickr. I tend to recommend photobucket because it seems more user friendly to me but now that Yahoo controls flickr, it seems to be getting better.
There may be others but those are the two that I use exclusively because they are free and allow multiple registrations.
You can also use this site as a host for your photos by using the "attachment" feature to post a photo in a post. See option #2 in
this post for step-by-step instructions.