Nikos said:
Are our perosnal files(photos) really safe stored in those cloud servers or they are accessed by others?
Any time you give something to somebody else to watch over, you're taking a risk.
Your files are only as safe as the companies security measures are.
You can take steps such as encrypting your files, but if you plan on sharing them, anyone you share them with would need your key to decrypt them.
As with anything computer oriented, usability reduces security.