That depends on the app or program in any computer, not just in Android. If you save the edited version as the original file name, you should get asked if you want to overwrite the original - assuming that the app you use for editing has a "save as" function. Otherwise it's up to the developer - always overwrite or always keep the original. (It holds true for text editors too.)
As more and more people who aren't really software developers "develop" Android apps, you're going to see more and more "you can only do it this way" apps - and that way will be the way the "developer" always does it. Fewer and fewer people are being taught to learn to look at things from the eventual users' viewpoints before starting to develop an app. (I guess that comes under the heading of "everyone should be able to develop apps" - it's about as true as "everyone should be able to play Mozart", but a lot of people have been misled into believing it.)