There are many programs that can batch-resize pictures: XnView is one.
The problem is that resizing is based on width or height, not megapixels. But it's easy to do some calculations, especially if your pictures have mainly the same ratio, e.g. 4:3, 3:2, ...
But the only site that allows upsizing (from 6 MP to 8 MP) is Alamy - all other sites explicitly forbid upsizing or allow a max 5% increase; all you will get is being banned.