Aviary is a weird old service. It’s a web-based photo-editing suite that runs in HTML (and therefore the iPhone and iPad), but there’s no actual Aviary site where you can upload images and fiddle with them.
Previously, the easiest way to get access was to go to Flickr, but since nobody uses Flickr anymore, that was kind of lame. Now, though, somebody has licensed the Aviary APIs and made an iOS app. Right now it’s iPhone-only, but it’s pretty damn good.