I compressed some pictures it a zip file. Next, I opened this zipped file with Notepad++, changed it a bit and saved it. As expected, in this condition the zipped file doesn't open up.
When I undo the changes I made to the zip file, it opens up and the pictures are still there!
My question is, is this a good method to secure my personal files? Most importantly is it possible that the changes of the code could be undone by a software or other ways?
No, it's not secure.
First, it relies on security through obscurity. Someone who knows you used that method might be able to undo it by trying random changes to the file.
Second, you don't know what you changed, so it is possible you changed something unimportant which might be fixable by a better packing tool.
Third, zip files can be password-protected. Why don't you just do that? (for further reading I recommend the question Is it easier to crack a ZIP file than a 7z archived file given they have the same password?)
That is a very insecure way to hide your files.
An ordinary zip program cannot restore this corrupted file; but a smarter program will be able to determine that this is a zip file. It will be able to determine that some errors are present in the file, and may well be able to compensate for them.
Data recovery programs will be able to restore the contents of the corrupted zip file. They may not be able restore it all, but they'll be able to restore a lot.
If you want to keep your files save, encrypt them with a reliable program using a good cryptosystem, and use a good password.
You are mixing up "security" and "obscurity". In the same way, you can hide your dollar bills somewhere in your house in a place that is not obvious to search for at all. Doing that, the "strategy" you're relying on is simply the hope that someone who searches for your money doesn't have the same idea you had when you hide it. And hope is not a strategy. When people are doing security they try to employ methods which allow to quantify the minimum amount of effort an attacker needs to use to break the protection. In your case someone just needs to debug an decompression program to see that it breaks in a certain point and take it from there. Doesn't really sound impossible, nor does it need millions of CPU or similar. Your "security" is just the hope that people give up if the normal zip programs say that the file is not a correct zip file ...
