And where would you get those critiques.?..

Patrick.

im trying to build up my portfolio and as i do im seeing i have little insight to what will be approved or rejected.  wondering how the ratio between refused/approved images affects my exposure on these sights. 
i generally upload everything i think is good but im wondering if being more selective (perhaps getting critiques first) is wiser; but there again this slows the process down incredibly.  advice?

Post what sites you upload to, cause diffrent sites have diffrent ways of doing things.

Id like to get a definit answer on Istock~i have been geting conflicting info, when searching for the answer on line.
If we have a very low acceptance rate does it effect our search placement, or anything else?

Without seeing the code for the proprietary image search results, there is no way to know how the approval percentage plays a role.  Dreamstime definitely includes the approval % in its formula, but to what degree we can only guess. 

The best advice I can give is to keep your approval percentage high everywhere, and focus on quality more than quantity.  A few strong images per month will probably outperform a lot of mediocre uploads in aggregate.  I'd rather have a portfolio of 100 strong/unique images than 1000 also-rans.  So take your time building things up, and don't be afraid to throw a lot of images in the trash at the onset rather than uploading them for the heck of it.  Let your expectations of your work rise.  I wish I had done this at the onset.  I think I'd be in a much better position today had I done so.

Which brings us to another potential ingredient in search engine formulas...

Download to Upload Ratio

Often theorized, but again no way we could ever know for sure if it is in there.  If it is, then quality over quantity is definitely the wave to ride.  My guess is that it is included.  If I had an agency, I'd make it one of the top criteria in the search results.