Upload & Sell: Off
The return is trivial. I made my living with my cameras for about a decade, so I'm not thinking out loud. I've looked at the numbers and it's a waste of time, even for a hobbyist. The return is ridiculously low. There is still money to be made marketing unique stock images (through full service agencies of on your own, not through microstock) but it takes a lot of time, legwork and some genuine insight and marketing savvy. Take the 'gone fishing' image on their main page. A standard license runs about $120. The default royalty is 15%, so each time that image sells, you get $18. Most hobbyists insist that it's "free money", 'cuz otherwise files are just sitting on your hard drive. Don't forget all the time (and therefore money) required to make the image in the first place, plus the necessary post processing, archiving, keywording, uploading and back-end work. Personally, my hobby is now making pictures, not keywording.
On a more personal note to Dorian - the images you post here seem sufficiently high quality to sell (can't say for sure without looking at the full-rez file, a 640x640 jpg can hide a lot of imperfections.) More importantly your pelagic birds really are unique in the market. I can't imagine that finding those species is easy, convenient or cheap - you need a boat, fuel, probably spend hours and hours fruitlessly looking with a companion (read: "assistant")or two. Don't forget the very specific expertise you've acquired in order to find them. These are all worth money to you and therefore a photo buyer.
Do yourself the favor of deciding whether you REALLY want to market your images. It will be work, you'll hear a lot of "no's" and run into a lot of brick walls and closed doors. It will be frustrating and disheartening. But if you do get serious and stay persistent about it, the images will be worth a LOT more than $18 to the right buyers.