I'm currently using AWS S3 credentials inside my application's code in a file ignored by git, but every developer has that file so it can work with images.
I want to hide those credentials from the developers, but still have the ability to use S3 in the development environment.
Give each of your developers IAM accounts with their own API keys, set your application to read the API keys from environment variables rather than from a config file. When the code runs on EC2 instances, use IAM roles so that you don't need to use API keys at all. If you're using AWS SDKs it will use role credentials by default.