Here's some shameless spam.

I have used here Snapseed* photo application for post processing. I wondered how come such an incredible mobile phone photo editor can be a free ware. Then I found out it's part of the package Google bought from Nik Software, and made it free to all users. So, thank you Google!

It is the first really functional and useful mobile phone software for image processing. It keeps the EXIF untouched, it is easy to use, and full of great features. Used along with also free and now unlimited** Google Images cloud service, it provides an excellent tools to work with images online.

The only Snapseed's limitation is that it does not let adding text small enough (e.g. watermark signature). Text can be added on the image, but even the smallest font size is way too large.

*Source: en.wikipedia.org/wiki/Snapseed

**On Google Images settings choose lower quality. Then there is no limit for how many images you can store to the cloud. Image quality is still good enough for amateurs like me, who shoots 8MP JPEG images only.