We have no clue how Google decides the ranking of pictures in Google Images but they are probably giving more preference to fresh content in image search results.
For instance, a search for “Google” on Google Images shows the following three pictures within the top 10 positions on the first page of results.
If you observe these pictures carefully, two of them were published just few hours ago and they are about Google playing the spoiler in the Microsoft - Yahoo! deal. The third picture (from NY Times) was published 3 days ago and already made it to the front page in the organic rankings for a competitive term like “Google”.
And this is not just a one-off case. If you search for Microsoft on Google Images, 5 of the top 10 search results feature news images that were released in the last 2-3 days.
The second observation is that images from news sources have a better chance of appearing at higher positions in Google Images.
Also read: How to Get Your Pictures on Google Images