I'm looking for a free, preferably open source Android image editor which supports basic functions such as cropping, resizing and rotating. I use an AOSP device with a Google Camera app so I would like it to visually and functionally integrate with it meaning "the same principle of using the app" on a subjective level - not that it needs to interface with the Google Camera app directly (the editor file association is handled by the Android itself). The support for batch processing would be another plus, but it is not strictly necessary. Thank you for your suggestions!
After extensive search I've found an acceptable solution and that is the AOSP Gallery app ported from Android 4.4 (which also includes basic image editing and filtering options). It can be found in the Play Store under the name Galery KK.