Just checking the qt.exploratorium.edu site and there's a very large number of pictures from Opportunity for June 14th, particularly of pancam shots. They seem to span several days of shooting.

Is this just clearing out the flash memory of the rover, or what?

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Huc accedit zambonis!
Here comes the Zamboni!

I assume that periodically they have particularly good opportunities (no pun intended) to do downloads from the rovers and take advantage of it. Keep in mind that there's considerable contention for time on the Deep Space Network stations from the Mars missions and (especially now) from Cassini.

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Everything I need to know I learned through Googling.

And of course notice the days before had no pictures in that site which supports your notion that it is a backlog issue.

The June 14 NavCam file has some really old image like this image of Eagle Crater.

Why was not that image taken care of a long time ago? I sure hope the memory is not getting clogged up with months old images.