When I first started using my iPhone and iPad for editing images, I came across the app Snapseed. This app is a Google product, and it's completely FREE! Here
are just a few of the amazing features of this app:
1. You can improve and tune just about every aspect of your photo... brightness, contrast, color, shadows and highlights.
2. Cropping, sharpening and straightening (great for those crooked horizons!)
3. Remove unwanted items from your photo.
4. Add text
5. Tons of great filters... Glow, Drama, Grunge, Vintage, Retro and Black & White to name a few.
And... that's just the basics! Watch for more detailed information and a tutorial on this app... but in the meantime, go ahead and download it from the App Store. Play around with it. I'm sure you'll be amazed!