Good question. Here's my two cents as an experienced author but new to HubPages - where I have no editor to help me...The way I have been doing it is to link to the page url that gives the sharing info along with the file. I do not really think it matters whether you link to the photo file or to the page, but I am relatively new to online writing and new to HubPages.
I also wonder whether we should include Wikipedia in the source ID or just the photographer if there is one noted. Lately I have been just including the photographer and not Wikipedia, my reasoning being that it looks better and I have already included the url.