Marty-
I really don't know what people prefer, but I post the photo, as I think it's much easier to just let the folks see it there, rather than have to click on it, open a new window, etc.
I think some just post the full link simply because it's either the easiest thing to do, or they don't know how to post the actual photo.
We do get lot of questions about how to post photos directly in posts, showing that everyone doesn't know how to do it.
The other consideration might be that those with limited connection speed or those viewing on older or smaller computer screens or smart phones may not prefer to have the photos themselves in the posts, so they don't have to sort through them if they don't want to see them themselves.
On Edit: I actually do quite a few posts where all I do is post the photo a person referred to in the previous post, so that it can be seen more easily. Most all of the folks who made the original post send me a PM or email, thanking me for doing so.
My 2 cents, YMMV, and probably does!
Joe. :teeth :thup