Starting today in our staging environment (http://apiblog.youtube.com/
2008/11/all-worlds-stage.html),you’ll find an additional thumbnail
entry listed for each video. The thumbnail is identified with the
“yt:name” attribute of “mqdefault” and its size is 320 x 180 pixels.
The new thumbnail will only be available in version 2 and above of the
Data API. The underlying XML element looks like
height='180' width='320' yt:name='mqdefault'/>
We plan on pushing this change out to the production environment
after it’s been live on stage for at least a week, so the earliest
that will happen is Wednesday, April 11.
Most developers will not have to do anything as a result of this
additional thumbnail—using it is entirely optional. However, we know
that there are some developers who access specific thumbnails based on
the order in which they appear relative to other thumbnail entries in
a video entry—for instance, your code might always access the second
thumbnail returned by the Data API, blindly assuming that it
represents the “hqdefault” image. If you have code that makes those
assumptions, you should remediate it prior to next Tuesday, or else
your code might inadvertently read the incorrect thumbnail from the
Data API response. Please test your code against the staging
environment now to see if any changes will be needed.
The recommended best practice is not to make any assumptions about
the order in which thumbnail entries are returned in Data API
responses, and instead rely on the “yt:name” attribute to identify the
thumbnail that’s best suited for display in your application.
-Jeff Posnick, YouTube API Team