yes, first ones need to be a competent photographer, and able to self motivate, self discipline to shoot stock oriented images, key wording, and uploading it to agency
some agencies will need to summit a few images for review before accepting you, some will need you to do paper test.
for istock, you need to take a quiz before you able to summit 3 samples for reviewing, if your initial images being rejected, you have to wait for one month to resubmit new set of samples images for reviewing again. have heard of people tried three or four times before being accepted.
anyway, even after being a contributor, does not mean all your following summing images will be accepted. each images will still subject to approval by reviewers.