Retain some control. If you upload photos of yourself, or friends/family with consent, it's worth going for the 'rights managed' licence option – otherwise you'll have little to no control over how your images are used (eg, you could star in an ad for haemorrhoid cream). See Alamy's page on understanding stock image licensing for more on the different types of licences.
I have not personally done this but I would like to eventually. If you are artistic you could make artwork or handmade good such as ornaments, wallets, or jewlery and sell them online. Something that I personally would like to do is design T-shirts and sell them. Shop for anything from creative people everywhere is a great site and I have bought many things from them. If you make quality products and get your name out through Facebook or other social media you could get a substantial business going.