What it means is that people that were registered before the change in mp standards can still upload 2.5 mp images but people that registered after the change have to upload 4 mp minimum.  You will never be able to upload less than 4 mp if this rule was already introduced after you joined.

I joined in January 2007 and can only upload 4MP ... bummer.

sure is, but others also require 4MP (fotolia, crestock). dreamstime min is 3MP, while BigStock is 800px, which i think is the most flexible.
i would think that one would prefer to UL the largest best, but at time you just don't have one that is 4MP.
pity, because i see some really lousy stuff out there yet they still turn down my stuff which are much sharper, more interesting.

the mind boggles when they write me to say, "we already have enough of these types, yet you look at what they have , they're truly lousy".

so where do you really draw the line.

sorry if this went off topic a bit.
but the stress should be the quality and the asthetic value of a shot, not how large it is.
don't we all agree?