Unlike Photoshop where you can do everything in one piece of software, apps on devices usually have a specific function and sometimes you might only use an app once every 3 months but at that point it's an app you might really need, like Handy Photo for example, which enables you to extend the side (or sides) of a photo, guessing what might be there. If you have a photo and need a bit more sky, you drag out the top border and it automatically fills in more sky if that is what was there in the first place (it also happens to do a good job of getting rid of objects in a photo you don't want amongst other things). However, there are some apps which do more than others.
Five Top Photo Apps
If someone said I could only have 5 photo apps on my iPad, these are the ones I would choose and I will explain later. A tuning/enhancement app (Snapseed - free - but I might go for Enlight because of it's lossless format), a sketch app with layers (SketchClub or many others would choose Procreate and I would too if I knew it better!), a painting/effects app (iColorama - no contest here), a selection/cut out app (difficult one but probably Eraser - but I use Superimpose and Photoshop Mix as much), a healing app to take away unwanted spots or objects (Handy Photo only because of it's ability to extend photos otherwise it would be Photoshop Fix which is free).
3 Free Apps that cover all your photo improvement requirements
If you only want to improve your images and not actually give them painterly looks you can get by with totally free apps: Snapseed, Photoshop Mix and Photoshop Fix.
Before I get started on the apps themselves in Part 3, I want to talk about image resolution. If your intention is to print your photo manipulations or sell them on the internet, you need to make sure they are saved in high resolution so they still look clear when you print them. Be aware that some apps will reduce resolution when they process your images so always check your image size after processing with an app using the ViewExif app mentioned in Part 1.
Also, every time you save a .jpg file, you lose some definition so if possible save your file as a .png or .tiff file right from the start. If I am going to do a lot of manipulation to an image and it has a low resolution to start off with I will increase it's size right from the beginning. Sometimes increasing the size distorts the photo but as I often make significant changes to the photo anyway this doesn't worry me. However, if you want to just 'tune' a photo a bit - increase the contrast or colour etc - there is a limit to how much you can increase the size without it distorting. You have to accept that it's just not going to be an image you can make a large print of and put on your wall.
Big Photo by Zynsoft Inc
In my mind this is the best app for resizing your image whether you want to make it smaller or larger. Also, it is another app I use to view information about my photo. However, I have saved a couple of blank canvasses at high resolution (using SketchClub or Procreate) and will often bring my photo into these to get the right size and placement - only if I am going to change it a lot though. Otherwise I will use Big Photo if I by chance am forced to do it at the end which I try to avoid. As I mentioned before resizing it considerably will cause it to distort because the app has to work out how to fill in the pixels and it doesn't always get it right.