Do HTML attributes need quotation marks?

I am working on a web site, and am wondering if you need to put quotation marks around HTML attributes for example I've seen code like:
<img src=http://ift.tt/HIc84i width=350px height=200px />
instead of:
<img src="http://ift.tt/HIc84i" width="350px" height="200px" />.
Does anyone know the answer to this?