Dreamstime is the second page I got into, in 2009, around the same time as Shutterstock, and they currently make up about 8% of my sales. They are considered easier to get into than Shutterstock, so it might be a good starter page.
Check their guidelines, you’ll see photos only have to be 3 MegaPixels in size. Look also at their editor’s choice of best photos page
When you are ready to submit your photos, I would appreciate you using this link. If asked for a Referral ID, please use res1430553 . It doesn’t cost you anything and I get a referral bonus and the pleasure of watching your progress! Thanks.