I don't quite understand how to make graphics for IOS. My problem is that I don't know what size to create the image in paint.net. I want my app to be universal. For example I want to create a character at the bottom of the screen, lets say that for the iphone 6. I want the pixel size to be (in portrait mode) 200px by 400 px. I can create that image in paint.net with that size but I also need to create two other images for higher resolutions. What sizes should the other two images be? Am I going about this correctly?
In your case, there are 3 sets of images you have to prepare:
There are other cases that you might want to take care of, especially background images, launch images as well as app icons.
To manage assets in Xcode, it is suggested to use Image Assets, which will advise you which image is missing, and which size is required.