Nothing to download or install, just go to the site and follow the instructions. I do recommend that you click on the "Tour" link, and watch the video which looks a lot like something the teenaged geek next door would make, but is helpful.
There are also some special effects that you can use for your photos, including brightening, darkening, sepia, and grayscale, which changes your photo to a black and white version. You'll find these under the "Filters" menu.
Simple and free are two good things when it comes to managing your photos and images, so I hope you find this helpful.