AppZapp by ConIT AG
Firstly, a lot of apps add up in terms of cost so if you want to save a couple of pennies download the AppZapp app which will tell you when apps are going free or cheaply - you can set up the app to contact you if an app you are interested in goes cheaply or free. Be warned it is addictive to pick up apps! Also, the Apps Gone Free site: http://appadvice.com/apps-gone-free
Let me start by encouraging you to use your own photos as much as possible in your art. It means you never have to worry about copyright and it really is your own art. If you go out one day with the purpose of just taking photos it's surprising how much you can find. I did a 365 day photo project and had thousands of photos at the end of a year that I could make use of. Keep your iphone at the ready and take photos whenever you get the chance. Even just photography stuff around your house like fruit, vegetables, crockery etc.
Here's a couple of jugs:
After in 15 minutes
Camera + by tap tap tap
A good manipulation almost always starts with a good photo. The iPhone camera app is very good but if you want one that enables you to have manual control and also enables you to save at high resolution look no further than Camera+. And if you want to understand all the controls watch Jack Hollingsworth's videos on Focus, Exposure and White Balance: http://snapsnapsnap.photos/author/jack/. Also, to get inspired and see what you can do with an iPhone watch his picture perfect photos on youtube Adorama TV: https://www.youtube.com/playlist?list=PLwm6LvGc5xq2TQKM3Iz10EIPe86o0ZvwG
Hipstamatic by Hipstamatic
Want to take retro shots or apply interesting old camera effects this is the one to go for. Check out Hipstography to see what you can do. The drawback is that after buying the App, you have to buy the different films and lenses to get the effects (combos). Start off with one or two of their retro packs to get going. Also in the new version you can change the effects you have applied or remove them completely.
Here are two photos taken with different effects in Hipstamatic.
Photo Transfer Pro by Bitwise
If you take a lot of photos on your iPhone backing up is an absolute must - don't delay - do it now! If you have a Mac, you can link up your iPhone and copy your photos straight across but if you have a PC, Photo Transfer Pro is a must. Also, you can transfer photos across from an android device to an iPad for example not just over wifi but using bluetooth (you can obviously do this between iPad and iPhone too - don't forget Airdrop for this also).
ViewExif by Steven Zhang
I can't understand why Apple doesn't make it easier for us to see info about our photos in the picture viewer. However, this app will show you all the info about the photo from within the photo viewer. Once you have installed the app, you can open it and it will show you how to set it up in the settings. Then you can go into the picture viewer, tap the share button (box with the up arrow), tap ViewExif and see all the info about the photo's settings as well as the name, size and date.