First bit of advice... easiest and fastest doesn't matter. 

It's all about ROI... Return On Investment.  I'd say Graphic Leftovers is easiest... simply use their web-based uploading window, upload a bunch at once, and no follow-up needed.  I think the team there is great, but most days, I have no sales.  They're a site to watch, though, so I keep uploading despite the low ROI at the moment.

Here's a better way to answer the question that might help you... who is fastest/easiest AND who delivers ROI.

I'll rate the sites that matter on a scale of 1 to 10... 10 being fast and easy, and high revenue... 1 being painful upload process, and pitiful sales...

Shutterstock:  Uploading - 5, ROI - 10

Fotolia: Uploading - 5, ROI - 9

iStock: Uploading - 1, ROI - 5

Dreamstime: Uploading - 5, ROI - 7

Bigstock : (I'm in the Bridge to Bigstock program, so I can't recall what they're like, but I'd say ROI is 4.)

123RF: Uploading - 7, ROI - 6

CanStock: Uploading - 6, ROI - 5

Veer: Uploading - 2, ROI - 2