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[MarvinCZ]

Before you tell me to use search engine to get them, this is a project, not home work. And such I cannot use photos that are copyrighted or not useable for public.
It will be avaliable to public and for distribution. So if you are fine with that and have your own photos to share , please do it.

What's the issue? You can find many free pictures using a search engine, you just need to check what you've found. Especially when you realise that most photographs from U.S. government agencies are free to use (check their sites for details) - like NASA linked above.

Edit: Even if you find some private photographs, just politely asking the author may be enough to get permission.