It seems that Shutterstock has increased the minimum size of an image from 2.5 to 4 megapixels.  Old photos have been "grandfathered" into the system and will not be removed.

See the following thread for more info:

http://submit.shutterstock.com/forum/viewtopic.php?p=104365#104365

Here is some more info from SS admin:

"The 4mp minimum is for newbies only. Technically, it only applies to newbies who created their submitter account after May 17. If you created your submitter account before that date, you can still upload <4mp photos; if it was after that date, you are bound by the new policy. No photo that has been accepted already will be removed at this time. We are in the process of updating the text of the site and FAQ to reflect the change."

I've had many sales from my 1.9Mpix camera's images (I have a 3.9Mpix now, hopefully getting a DSLR later this year).  1600x1200 pixels images make excellent 10x15cm prints and very good 13x18 ones.  This covers a lot of uses, even in magazines (not full page images of course).  I've sold many smaller versions as well in IS (800x600). 

So while I understand higher MPix cameras are more common now, the use for low res images is still wide.  Thus, limiting these lower res cameras also restricts the contribution of many good photographers.  In fact have great 640x480 images from Peru, Bolivia, France and Iceland that I can't sell at these sites, though they would be totally acceptable for website use (travel sites, for instance).

I haven't joined SS because I think 0.25 per full-size image is ridiculous.  Now it's even worse. 

Regards,
Adelaide