So, as most iOS developers know, it a pain in the ass having to get all the images and their dimensions. I am currently using quite a handy tool called Resizer. The app will resize a 2x image to a standard image size, so then you only have to deal with the 2x images, you will be surprised with how much time this has saved me.
Sometimes when submitting an app to the app store, you struggle to get some good images. I came across this website, its pretty cool, you can upload a picture of your app and then it will appear on a premade photo but on the iDevice of your choice. Check it out.