I came across this app today and I’m glad I did. I love how easy it is to use. Normally, its $1.99, but it’s free right now.
The one thing you NEED to know is you have to hold your finger on the camera or photo buttons to work on hi resolution images. If you only tap on them, your image is no larger than 1024×1024, like these 2. I did notice the brush strokes were slower, while working on hi res images with the iPhone 5s, but hi res is better than 1024×1024.
If you tap on more, there are 9 brushes to choose from, as well as an opacity slider. This is a good amount of customization, while keeping things very simple.
Go grab it here and have fun.