They're bots. Bots are your friends. They are sent from search engines to sniff out our pages, catalog them and hopefully send visitors here. From the FAA FAQ --
1:I see the same visitor visiting all of my images shortly after I upload them. Should I be worried?
No. That's nothing to worry about. There are lots of automated programs (called web bots) that scan the internet for new webpages.
If you see a lot of visits from the same location, it's just a web bot. Don't worry - web bots are good. Google uses web bots to add new pages to their search results. So do Yahoo, Bing, etc...
http://fineartamerica.com/faqlinks.html?id=14