I have a HTML5 website with some pictures and some data about the pictures. I would like to enhance their description so machines would be able to read them as well.
Say, there is a picture of Mona Lisa by Leonardo da Vinci, completed around 1503 - 1519 (n.b. "around" and "range")
What format should I use? What are the best practices in 2012? RDF? RDFa? Microformats? Dublin core? Schema.org?
What is the best way to integrate such metadata into a HTML5 page?
How to express a date which is not very precisely known?
RDFa is your best bet for embedded the RDF in an HTML page, that's what it's designed for. Beyond that, there's no schema you have to use. Something standard like DC makes sense, but I suspect there are also some image-specific schema lying around out there that you could make use of if you want to annotate specific details of the image, width, height, resolution, etc.
I don't have a good suggestions for dealing with an imprecise date. At worse you could annotate it w/ the start and end dates and call it a day. Depends on what you want done with that information. If you're going to reason with it, or expect someone else to, you'll want to look for an ontology about time. Otherwise, you don't need to get too fancy. You could always coin new URI's and when you scrape in the RDF do something sensible when you see those new properties.