MicrostockGroup

A little bit above 8 MP.
My Canon 30D produces 8,2 MP and needs a tiny little bit of upsizing.

You can easily calculate:
They want 24 MB uncompressed, i.e. 24 x 1024 x 1024 = 25.165.824 Bytes.
As an (uncompressed) 8-bit Jpeg needs 3 Bytes per pixel, you end up with 25.165.824 / 3 = 8.388.608 Pixels.

And as camera makers normally mean exactly one million pixels when using "Megapixel" (wheres in computers when talking about kilobytes or megabytes you are using powers of 2, so one kilobyte = 1024 bytes, one megabyte = 1024 kilobytes) , your camera would need 8.4 MP so you don't have to upsize for the 24 MB uncompressed size.