Using pictures not created by yourself allowed? XNA

Using pictures not created by yourself allowed? XNA

I was just wondering if you are allowed to use any pictures that are not
originally created by you, i.e. pictures from Google and/or pictures that
you are not the original owner of. I have looked this up before and found
that Copyright is a possible issue, which was obvious, so if there is a
company name next to a picture on Google, you are advised to read through
their terms and conditions and/or get their permission regarding using
their content.
I don't know if there is more to this or what if there is anything i
should worry/not worry about regarding using images from Google in my XNA
game.
Thanks for reading.