iOS: When it comes to adding annotations to images on iOS, what works best for you really depends on what you're trying to convey. If you're looking for a dead simple approach without a ton of flair, This by Tinrocket is an app that works well. Unlike an app like Skitch, which uses big, bright, bold lines, This by Tinrocket takes a more minimalist approach. You only have one font choice, two font colours, three font sizes and seven different pointer options. That simplicity makes it easy to add labels, descriptions or whatever other commentary you want to photos. It's probably not a thing you're going to use all the time, but if you're looking for an annotation tool that's a little more simplified than other options, This by Tinrocket is a solid option.
This by Tinrocket ($2.99) [iTunes App Store]