If you are on a mobile, then the best is PicsArt http://picsart.com, also you can directly post to facebook when you have done the necessary edit to your photos!
There are many online editing programs (and smart phone apps) that would do the trick.
Here is one I found through a quick Google search.. http://www.fotor.com/.
I personally use Photoshop on my laptop (they have an inexpensive monthly subscription price you can pay) or Over on my iPhone.
Good luck!